Enable AsciiDoc markup in default settings.

The default settings.py currently only enable reStructuredText and Markdown markup readers. This means any AsciiDoc input files will be ignored silently even if AsciiDoc is available unless 1) a user explicitly sets MARKUP=('asc',) or 2) set the --markup flag, which is currently problematic because of the bug https://github.com/getpelican/pelican/pull/898. This doesn't sound like a very reasonable default behavior, especially since Pelican has no good reason to discriminate against AsciiDoc in particular in this respect.

This commit adds 'asc' to the MARKUP tuple in default settings to enable AsciiDoc by default. It also changes the description of the --markup flag to include 'asc' as an option.