join*() functions to compliment the split*() ones

[bradleythughes]

While doing SDP munging, it's handy to use SDPUtils to split things up and find the pieces you want to modify before putting it all back together again, so I wondered if there should be some helpers for doing so (putting an SDP back together).

[fippo]

the main use case I am interested in here is this one:

return SDPUtils.joinSections(SDPUtils.splitSections(sdp).map(section => ...))

[fippo]

do we need joinSections and joinLines? Or just join()?

[bradleythughes]

For API symmetry, joinSections() and joinLines() maybe makes sense... but if the implementation is the same for both... I dunno :|