Add option to change title format

[E1k3]

This PR adds the option to change the report title format as discussed in #385.
I added the option to HtmlReporter, TextReporter and MarkdownReporter, because all of these currently use the "pretty (location)" default format in some way.

I tested the text and markdown reporters and they work like discussed. I couldn't test the html reporter, because I couldn't get my mailserver to accept my smtp password for some reason.

For now, every setting other than "pretty" and "location" falls back to the default format.
If you want an error message for unknown title format specifiers instead, let me know.

[thp]

The cfg.get() call with a default is great for existing configurations that lack that entry.

New configs should however have title: default in there, this makes it also a bit easier to discover this feature (add it to the relevant entries of DEFAULT_CONFIG in storage.py).

Since the gotify reporter already uses title, and we might want to use title for something else in the future, AND you already call it title_format in the Python variable that gets used (for good reason: it's the title format, not the title itself), I'd suggest we call it title_format in the config as well.

Also, there should be added documentation for this feature, so that users can find out about how it works and what possible values exist for title_format and to which reporters it applies.

Another thing I would do to detect misconfigurations (instead of accepting them silently):

if title_format not in ("default", "location", "pretty"):
    logger.warn(f'Invalid title format {title_format!r}, falling back to "default".')
    title_format = "default"

Otherwise, if I set title_format: foo, it will happily accept this without complaint, and use the default format. If a future version adds e.g. "fancy" as a title format, and someone tries to use it, but uses a version that doesn't have that title format supported yet, instead of silently falling back to "default", it will still fall back to "default" (a sensible behaviour), but also write out a warning, giving a clue to the user that the program has done the fallback, because it doesn't recognize the value set.

[E1k3]

I added the default options and the warning.
Regarding the documentation: As far as I can see, there is currently no place where the Text, Html and Markdown reporters are individually documented.
The Markdown reporter is somewhat documented in the Matrix reporter section and the Html and Text reporter options are shown without explanations in the Reporters section of the Configuration page.
I can add the title_format documentation to these locations or create new sections on the Reporters page for all three reporters.
What do you prefer?

[E1k3]

@thp I added the format options to the places I mentioned. If you would like more elaborate documentation of the title_format feature, let me know where in the docs that would make the most sense.

[thp]

@thp I added the format options to the places I mentioned. If you would like more elaborate documentation of the title_format feature, let me know where in the docs that would make the most sense.

Probably a section in the https://github.com/thp/urlwatch/blob/master/docs/source/advanced.rst page, something like "Customizing the title format in reports", with all available options and examples what they do.

[thp]

@thp I added the format options to the places I mentioned. If you would like more elaborate documentation of the title_format feature, let me know where in the docs that would make the most sense.

Probably a section in the https://github.com/thp/urlwatch/blob/master/docs/source/advanced.rst page, something like "Customizing the title format in reports", with all available options and examples what they do.

@E1k3 Adding the docs would make this PR merge-ready, I believe.