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"'...

.travis.yml

@@ -1,4 +1,3 @@
 sudo: false
-install: git clone --depth 1 https://github.com/sstephenson/bats.git
-script: PATH="./bats/bin:$PATH" test/run
+script: test/run
 language: c