Implemented auto-linking of local function definitions.

Eg. `test/1` => [`test/1`](#test/1).
Only linking functions defined inside code markers with an arity (`test/1`).
Not linking functions that are not defined in the current module.
Not linking functions that are already wrapped as a markdown url (except when the function doesn't touch one of the brackets, in which case the lookbehind/lookahead will fail to detect the brackets).