new: added ``body_process`` and ``subject_process`` options. (fixes #22)

These options superseeds ``replace_regexps`` and ``body_split_regexp``
as they provide a full control over text transformation of the subject
or the body of the commit before they get included in the changelog.
fix: update the documentation about using ``!`` instead of ``@`` !minor
chg: small cosmetic fixes here and there !minor
fix: bogus messages when template didn't exist.

Refactored out the common code and corrected the bad error message.
chg: dev: more output intelligence moved towards templates !refactor
fold error msg
new: dev: added a new test to run at least once all provided templates.
new: added 'octobercms-plugin' mako template. (fixes #16)
new: provide support for older config file format.