Add filetime_from_git plugin

Many people are using git to manage blog contents. Setting meta ``date`` in every article and page is a dirty work (and may often make mistakes), the core method ``DEFAULT_DATE = 'fs'`` may potentially be broken by copying/moving/touching files.

When this plugin is enabled, it will assign two fields into metadata, ``date`` and ``updated``, determined by git commits (if the blog is not managed by git, it will ignore silently). It overrides the original ``date`` set by article/page meta or pelican-core from fs time.

This module requires python package ``gitpython`` installed.