added support for raising KeyboardInterrupt

[fearphage]

Background

fzf throws 130 to signal when it receives the SIGINT signal.

Change

This change converts that special exit code into raising a KeyboardInterrupt exception. This allows users to recover cleanly from SIGINT when using iterfzf. The below currently does not work, but it will after this PR.

try:
    result = iterfzf.iterfzf(...)
except KeyboardInterrupt:
    print('caught ctrl-c')

PS. I tried writing a test for this, but I ran into some issues.

[dahlia]

Thanks for your contribution! Looks good to me.