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"'...

completion/available/bash-it.completion.bash

@@ -53,7 +53,7 @@ _bash-it-comp()
 	prev="${COMP_WORDS[COMP_CWORD-1]}"
 	chose_opt="${COMP_WORDS[1]}"
 	file_type="${COMP_WORDS[2]}"
-	opts="help show enable disable update"
+	opts="help show enable disable update search"
 	case "${chose_opt}" in
 		show)
 			local show_args="plugins aliases completions"