Move xolox#shell#execute() -> xolox#misc#os#exec()

Most of the Vim plug-ins I write have to execute external commands;
providing them input, intercepting stdout/stderr, verifying the exit
status, etc. Naturally I'd like to implement this in one place where
I can easily call it from all my plug-ins, even when vim-shell is not
installed. This is why I'm moving most of this code into the
miscellaneous script (which will be merged after this commit).
Make xolox#misc#os#exec() report exit codes (breaks backwards compatibility)

Working towards fixing this issue: xolox/vim-lua-ftplugin#8
Updated miscellaneous scripts