Standardize all predefined searches to use Ctrl

[m1foley]

Reasons:

• The Alt key is inconvenient/confusing for most OS X users. Alt is not enabled by default so it needs to be specifically remapped in a terminal application.
• Consistency is better. It's easier to remember the search keys if they all use Ctrl.

Why the Alt mappings exist:

• I assume there was a good reason to use Alt when the IP address search was added, but the commit message doesn't mention what it was.
• The SHA-1 search was originally changed from Ctrl to Alt because prefix + C-h conflicts with tmux-pain-control. C-s doesn't work either because it sends the XOFF signal, so changing this will require an awkward letter. I went with "a" because it's the vowel in both "SHA" and "hash".

Backwards compatibility with the old mappings should be added as well, but I'd like to hear your thoughts on this PR before writing more code.

[nguyenj]

Until this gets merged, here's my work around

# .tmux.conf

# set this before the plugin
set -g @copycat_has_search 'C-a'