Allow not indenting the test failure message

[jfmengels]

In elm-review's testing library, I try to print out helpful failure messages when a test fails. Sometimes that includes code extracts.

    FIXED CODE MISMATCH
    
    I found a different fixed source code than expected for the error with the
    following message:
    
      `Let value was declared prematurely`
    
    I expected the following result after the fixes have been applied:
    
      """
        module A exposing (..)
        a b c d =
          let
            y = 1
    
          in
          if DIFFERENT then
            let
                z = {a = 1}
            in
            {z | a = 2}
          else
            {a = 3}
    
      """
    
    but I found:
    
      """
        module A exposing (..)
        a b c d =
          let
            y = 1
    
          in
          if b then
            let
                z = {a = 1}
            in
            {z | a = 2}
          else
            {a = 3}
    
      """

I would love to be able to have users (including myself) copy-paste the code excerpts from the failure message, unfortunately, they are indented. Half because of me (to separate from the rest, but I could remove this), but mostly because elm-test indents all failure messages by 4 spaces anyway.

I would like to be able to have the option to disable the indentation, possibly by specifying it as an argument on Expect.onFail (and eventual friends from #239).

Here's an example of a potential API :

some operation
  |> Expect.equal x
  |> Expect.onFail { indent = False } "Some explanation..."

Note: From a rough search, I think the indentation happens in the CLI (here), so it might be necessary to modify the CLI as well.