Fix for #1381: Loosened regex definition of Gist ID to match hexadecimal format in use now, as well as any string without whitespace.
Fixed pre-existing bug breaking use of ReST-style syntax in Markdown when specifying a Gist ID without a file-name, added doctest to cover this case.
Merge pull request #1395 from mrabbitt/feature/git_id_format_fix-1381

Fix #1381: Loosened regex definition of Gist ID to match hexadecimal...