This gets us closer to having an API that can
be used my mobile.
The parse() function becomes a subset of
apply_markdown() that is no longer coupled
to the shape of a webapp object, and it can
be supplied with a new helper_config for each
invocation. Mobile will likely call this directly.
The setup() function becomes a subset of
initialize() that allows you to set up the
parser **before** having to build any kind of
message-specific helpers. Mobile will likely
call this directly.
The webapp continues to call these functions,
which are now thin wrappers:
* apply_markdown (wrapping parse)
* initialize (wrapping setup)
Note we still have several other problems to
solve before mobile can use this code, but we
introduce this now so that we can get a head
start on prototyping and unit testing.
Also, this commit does not address the fact
that contains_backend_only_syntax() is still
bound to the webapp config.
