Added option to parse markdown files and extract their titles

[trousev]

Now generator is using markdown filenames to guess menu title.

This PR introduces an option to use title in markdown file instead.

USAGE:

docsify generate ../docs --markdown-titles

it will parse every markdown file during generating sidebar and extract title. If there is no titles, filename (the old way) will be used as fallback

[1337cookie]

This works well on Win10 and Ubuntu, the merge conflicts with master are trivial. Is there anything else holding this back?