The goal of this change is to reduce number of non-standard library
packages (repositories) that blackfriday imports from 1 to 0, and in
turn, reduce the cost of importing blackfriday into other projects.
Do so by documenting the algorithm of SanitizedAnchorName, and include
a copy of the small function inside blackfriday itself. The same
functionality continues to be available in the original location,
github.com/shurcooL/sanitized_anchor_name.Create. It can be used by
existing users and those that look for a small package, and don't need
all of blackfriday functionality. Existing users of blackfriday can use
the new SanitizedAnchorName function directly and avoid an extra
package import.
Resolves#350.
