Extend footnote insertion with text notes

[iwahbe]

Description

Allow insertion of footnotes tagged with text instead of numbers.

This behavior is controlled with a variable: markdown-footnote-default-type.
Because of the default value set, this is not a breaking change. Changing the customization or
wrapping the function markdown-insert-footnote allows this to be useful.

I also introduce a customization to recalculate markdown-footnote-counter on each insertion.
I mostly work on small files, and delete footnotes a lot. This makes life easier. Again, by default it
does not change behavior.

Related Issue

None I'm aware of.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• Improvement (non-breaking change which improves an existing feature)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• I have read the CONTRIBUTING.md document.
• I have updated the documentation in the README.md file if necessary.
• I have added an entry to CHANGES.md.
• I have added tests to cover my changes.
• All new and existing tests passed (using make test).