[vim mode] Added gj/gk, :noh, defineEx(). Better :{num}, <{Key}> handling

- gj/gk for moveByDisplayLines motion, allows vertical movement within
wrapped lines
- :noh[lsearch] is now an ex-command
- new ex-commands can be defined through api, using
CodeMirror.Vim.defineEx()
- :{number} should now move to line with {number} on the gutter
- parseKeyString now also accepts keys like <Space> (in addition to
<C-{Key}>)