Added support for autodoc --templatedir option

[itsmoosh]

Adds handling for setting apidoc_template_dir in conf.py, which passes this variable along with the --templatedir command line option for sphinx-apidoc.

This enables the user to define custom templates, which is supported in sphinx.ext.apidoc but was not previously supported in sphinxcontrib/apidoc.

[stephenfin]

How does this work if a templates directory doesn't exist. Looking at the extension source, the --templatedir argument defaults to unset. Shouldn't we do the same?

[itsmoosh]

If the templates directory does not exist, the argument is ignored. I don't see a benefit to defaulting to unset. Considering the conf.py generated by sphinx-quickstart includes a definition for template_dir (https://github.com/sphinx-doc/sphinx/blob/35965903177c6ed9a6afb62ccd33243a746a3fc0/sphinx/templates/quickstart/conf.py_t#L38), I think users are likely to expect that the same default directory will be used for apidoc templates too. This was my expectation coming in. I made this PR because I was unable to include overwrite the apidoc default templates without editing the source in my site-packages. I think defaulting to 'templates' will save users some headaches.

[stephenfin]

I don't think we should be providing our own defaults that override the Sphinx ones...

[itsmoosh]

I don't think we should be providing our own defaults that override the Sphinx ones...

The apidoc templates are named module.rst_t, package.rst_t, and toc.rst_t. None of the Sphinx templates have these names, so we are not providing any defaults that override any others.

My motivation for defaulting to 'templates' is about providing expected behavior for users. Including this as the default will not create any conflicts, because the directory is neither created nor populated by default. The user must place templates in this directory in order to overwrite the defaults, so if they have done so they have already intended to alter the default behavior.