Improve the UI

This commit changes 2 things:

1) cleaner user interface
2) add 'retry' option even when the provided submission was correct
Merge pull request #110 from mhinz/master

Add 'retry' option for correct solutions
Make shell handle ^C/^D gracefully
Merge pull request #111 from mhinz/master

Make shell handle ^C/^D gracefully
Raise Interrupt instead of StandardExit

This fixes a bug when using Ruby 1.8, since it has no definition of
StandardExit:

"uninitialized constant VimGolf::CLI::StandardExit (NameError)"

Since Interrupt and StandardExit executed the same code when caught, we
just replaced all occurences of StandardExit by Interrupt.

The bug was introduced in 103e5279281bb3c6deed76f7b9c6d1558d74565e
Merge remote-tracking branch 'mhinz/master'
Write all input/output files correctly