Add support for escaping trailing hashes in headers

[facelessuser]

Originally reported in MkDocs issue mkdocs/mkdocs#1693.

[waylan]

I haven't tested it, but I'm assuming the regex should be modified as as follows:

- RE = re.compile(r'(^|\n)(?P<level>#{1,6})(?P<header>.*?)#*(\n|$)')
+ RE = re.compile(r'(^|\n)(?P<level>#{1,6})(?P<header>.*?)((?<!\\)#)*(\n|$)')

[facelessuser]

Yup, I was thinking something similar. I'll run it through some tests and adjust if needed.

[facelessuser]

The pattern suggested above unfortunately still has issues: r'(^|\n)(?P<level>#{1,6})(?P<header>.*?)((?<!\\)#)*(\n|$)'. But the truth is I was expecting this issue just based on my experience with similar situations in the past:

Most likely, I will use something like this: (^|\n)(?P<level>#{1,6})(?P<header>(?:\\.|[^\\]+)*?)#*(\n|$):