Add a main file and use it as a standard entrypoint

This moves away from having the script directory and
moves to the more standard console_scripts instead.
Add a utility helper module

This module contains useful helper functions.
Add a parser module

This module contains a parse method and a parsed
file that provides attribute access to the needed
components of that file (used during checking for
doing various functionality).
Gather files before running

Gather all the needed files before running so that
it's ensured that all files are ready to be used during
checking.
Add a checks module

This module contains the various checks that we include
by default in doc8 and the base classes these checks
inherit from.
Show the default max line length
Add config as constructor param
Move over the rest of the checks

Get the rest of the checks working (the
max line length check now works).

Remove the old script now that it is not
needed anymore.
Fix some flake8 warnings
Bump for 0.3.0