Support commenting out of imports

When GHC suggests redundant imports, the question and answers are now:

The import line `Foo' is redundant. Remove?

- y: yes, remove
- n: no, don't remove
- c: comment out the line

This is handy for easily adding back imports when they become relevant
again.

@hvr Might be of interest.
Quick hack to remove "Compilation failed" line

A better solution would be to use markers for these, but I'll prefer
to do that as a general update to interactive-mode when I've more
time. Everything can use markers to get rid of the ugly regex hacks.
Small indentation fix
Support jumping to filenames inside strings
Fix undefined reference to shm-display-quarantine in haskell-present