Allow to specify multiple components

[hasufell]

This doesn't work properly yet.

• roughAlgorithm works, it's the easiest due to splitAtPositionLine
• niceAlgorithm doesn't work, it seems my offset logic doesn't play well with splitAtPosition... I wasn't able to figure out why yet
• fancyAlgorithm: haven't tried yet

My idea was:

• don't re-parse the cabal file, instead parse and insert "as we go", because usually we only care about a specific line and everything coming after it... so we just need to compute the correct offsets for the next component and then execute the next iteration

An alternative I tried was: using the original cabal file bytestring for each component, then returning position + bytestring-to-insert and combine that in one pass. But here we also need to compute offsets, which is the main difficulty.

[Bodigrim]

I have not seen performance bottlenecks with Cabal parser, so I'd keep it simple and just loop in Main.hs over components re-parsing every time.