strings/json: Add string delimter match, and object create with vars methods

Add a function to check if there is an exact match a one string between
delimiters in another string.

Add a function that will create an ast_json object out of a list of
Asterisk variables. An excludes string can also optionally be passed
in.

Also, add a macro to make it easier to get object integers.

Change-Id: I5f34f18e102126aef3997f19a553a266d70d6226