Command.RawArgs has surrounding whitespace trimmed

[FiskFan1999]

Hello tulir and gomuks contributors,

This PR trims the extreme left and right whitespace around command.RawArgs, which, among other things, is used as the path of the file in the /upload command. I found the following behavior when using gomuks in macos: in the terminal, you're allowed to drag and drop a file from the desktop or "finder", and terminal pastes the absolute path, followed by a space. This caused gomuks on the latest master branch to throw a file not found exception, while taking care to delete the extra space character succeeds of course (steps to reproduce: in any room, type /upload /correct/path/to/file followed by a space, . This should fail and throw a "file not found" error).

strings.TrimSpace only trims space on the extreme left and right. Since strings.Fields also trims this whitespace as well as whitespace between the words I think it is reasonable to trim whitespace for command.RawArgs. As far as I can tell, this doesn't break any other commands that RawArgs is used for.

Thank you, am looking forward to your input on this.

[pataquets]

@tulir: Anything else needed to merge this in?