Up to now g:clipboard["copy"] only supported string values invoked as
system commands.
This commit enables the use of VimL functions instead. The function
signatures are the same as in provider/clipboard.vim. A clipboard
provider is expected to store and return a list of lines (i.e. the text)
and a register type (as seen in setreg()).
cache_enabled is ignored if "copy" is provided by a VimL function.
07ad5d71ab