Don't use user completion for completing unicode chars

Instead, make it always possible to complete using <C-X><C-Z>
so it won't break any other completion plugins.
skip over whitespace when using <Plug>(MakeDigraph)
Better swapping function
setup map to swap completion of names
Remove Enable/Disable commands
updated documentation
warn when it might be slow
better preview window output when completing unicode chars
stop slow running completion function
make sure, sort by value
fix annoying new line when pressing <f4> in visual mode
updated changelog
updated documentation
More documentation updates
added usage.txt file
moved function to plugin file
make function shorter
refactored Digraph code
adjust formatting
adjust comment