Add `markdown-beginning/end-of-line` functions and `markdown-special-ctrl-a/e` variable

[kuranari]

Description

Add functions to move to the beginning and end of lines (markdown-beginning-of-line and markdown-end-of-line), and the variable markdown-special-ctrl-a/e, like Org mode.

I implemented it based on this version of Org mode.

Related Issue

https://github.com/jrblevin/markdown-mode/issues/811

Type of Change

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

Checklist

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

[syohex]

Thanks