Add bash-it search functionality

- "bash-it search term1 [term2]...."

 - we are using existing 'bash-it show plugins|aliases|completions'
   commands output, to search (with grep) for lines that match the
   search terms, and then output the matches.

 - wrote a simple unit test that for whatever reason fails on Travis,
   so wrapped it in 'if "Darwin"'...

.gitignore

@@ -1,3 +1,4 @@
+.atom-build.json
 */enabled/*
 .DS_Store
 custom/*
@@ -9,3 +10,5 @@ lib/custom.bash
 plugins/custom.plugins.bash
 *.swp
 .*.un~
+bats
+.idea