Allow for configuring the number of lines of source snippet around a label

[brendanzab]

This would allow us to give users more context of problems.

Here is an example from Reason:

[Johann150]

Is this related to #125 ?

[elkowar]

Would be great if this was implemented. Is there anything specific holding this back? If not, I'd be open to trying myself at a PR for this

[elkowar]

I wrote up a mock implementation in elkowar@fa4d5f8
This is most likely a pretty bad way to do this, although I'm not sure how i'd improve it - I'm really not familiar with the codebase. Any hints and thoughts would be appreciated!

[Johann150]

Hmm, I think that would be the place to implement it, but not sure if its the best way, will have too look more into the code base again. But for a proper review you should open a pull request!

[Johann150]

I think this issue was resolved with #331