Fix headers at regex lesson
More reformatting indentation
Fix MATLAB challenges numbering
Comment optional item at YAML header
Merge pull request #777 from r-gaia-cs/optional-yaml-header-element

Comment optional item at YAML header
Merge pull request #768 from r-gaia-cs/fix-matlab-challenges-numbering

Fix MATLAB challenges numbering
Merge pull request #723 from r-gaia-cs/more-fix-at-regex-lessons

More fix at regex lessons
Index for web data tutorial
Merge branch 'master' of github.com:swcarpentry/bc
Merge branch 'gh-pages' of github.com:swcarpentry/bc into gh-pages