Merge pull request #69 from lyokha/master

folding: do not take embedded codes as folding elements, plus minor fixes.
don't show error messages if the menu doesn't exist when unloading it

should fix issue #71
make default mappings optional.

hypertext: use gf and gx for opening "links", like in netrw.

ref: issue #72
keyboard: override section movements.

also, update documentation.

should fix issue #72.
markdown/headers: prevent recursion limit being reached.
hypertext: allow special chars in urls.
keyboard: make ]] and [[ allow counts.

also, ]] and [[ are no longer optionally overridden and are *not* the
same as <localleader>hn and <localleader>hb.
keyboard: add [] and ][ motions, with counts

also, added markdown/sections module, to handle section-related stuff
not handled by markdown/headers.
folding: use a fdc of with 1 by default.

this is configurable through the g:pandoc#folding#fdc variable.
keyboard: implement section text-objects.
keyboard: split the module to help manageability.

updated the documentation.
keyboard/styles: implement span text objects.
pass extra input arguments into pandoc if any