Add VimuxRunLastCommandForced command

Add a new command: VimuxRunLastCommandForced, which behaves the same as the existing VimuxRunLastCommand - except that it invokes VimuxInterruptRunner before executing the command.

I have vimux setup to run whenever I save a buffer (using `autocmd BufWritePost * VimuxRunLastCommandForced`), which is often invoked more frequently than the time it takes the tests to run. Without this command I was having to wait for tests to complete before saving, or having to resave to run the tests.

Another good use case is for when the executed command never completes, an example being running a web-server.