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"'...

test/run

@@ -1,4 +1,6 @@
 #!/usr/bin/env bash
+PATH=$PATH:$(pwd)/bats/bin
+set +e
+[[ -z "$(which bats)" ]] && git clone --depth 1 https://github.com/sstephenson/bats.git
 set -e
-
 exec bats ${CI:+--tap} test/{lib,plugins}