Introduce ignore region customize variable

[syohex]

Description

The variable name is same as org-mode's one

Related Issue

812 (CC: @snan )

Type of Change

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] Improvement (non-breaking change which improves an existing feature)
• [s] 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).

[snan]

Hi, thanks for taking a look at my PR! ♥ I've found a problem with mine (C-c C-s ! with the cursor on the line of a header no longer works) but there's a huger problem with your variant here:

(defsubst markdown--region-active-p ()
   (and (not markdown-ignore-region) (use-region-p)))

That's the opposite of what I need. I don't want to ignore the region, I want to use the region even when it's not active! Normal, old school regions! That's why I had or instead of and, because I want it to work even when (use-region-p) is nil due to the region not being active.