idea - template debug comments

Migrated issue, originally created by jvanasco (@jvanasco)

i have an idea for template debugging that I think might prove useful -- "debug comments"

mako doesn't print line comments in this format:

## line comment

i propose a second comment form, something like :

##! debug comment

if the template (or template lookup) is configured with "debug_comments=True" , then that line (excluding the prefix) would be rendered. otherwise it will be stripped.

the rationale for this, is that it often makes sense to have extra html comments in a development/staging machine, but not on a production environment.

if this sounds good, and a prefix can be decided on, i think i can implement it and submit a PR. (though it'll happen after my sqlalchemy workload)

Review the template and template lookup configuration paths, focusing on how comments are parsed and how options such as debug_comments could be passed. Decide the comment prefix and the behavior when debugging is enabled or disabled, then add coverage for both rendering cases and confirm the proposed syntax works.