Fix small misspelling: Mardown -> Markdown
Merge pull request #883 from seanfisk/fix-small-misspelling

fix small misspelling: Mardown -> Markdown
Shows keyboard shorcuts overlay on pressing question mark
creates a new branch for the feature
Merge remote-tracking branch 'upstream/dev' into keyboard_shortcuts_overlay
Merge branch 'keyboard_shortcuts_overlay' of https://github.com/navateja/reveal.js into dev
merge and tweak key shortcuts overlay #943
adjust list of displayed key shortcuts #943