Add spelling movements

Resolves spiiph/vim-space#17
Documentation for spelling movements ]s [s ]S [S
Allow user-defined mappings

Allow to replace <Space>, <S-Space> and <BS> individually. The defaults
stay the same.
Add an option to disable the "non-GUI" mapping (by default, <BS>).
Clean things a bit: now gives a proper message if a key is already mapped.
Minor fix in conditional
Use echosmg to display 'already mapped' warnings
Revert "Use echosmg to display 'already mapped' warnings"

This reverts commit 8236f08efe2933bda796f322c307df71c0bd9297.
Revert "Minor fix in conditional"

This reverts commit 34d236b345fd4fd1d59c8d796b49727f742f53c3.
Revert "Allow user-defined mappings"

This reverts commit e206e20c0cb04fc8fced6fe6f7cf3f36ff117a16.
Unify how global variables are defined.

Also correct the behavior of g:space_disable_select_mode and
g:space_no_foldopen, which was broken when their value evaluated
to true.
Allow custom mappings for do_space
Add a g:space_no_second_prev_mapping option

This lets the user disable the non-gui mapping