Split display name at colon before generating temporary file name

This fixes an issue which occurs under the following circumstances:

* Sublime Text is installed on a Windows machine.
* On the remote system, rmate is called using a filename from the current working directory.

In this scenario, the display name generated by rmate is of the form "hostname:filename"; because there are no slashes to split on, the temporary file name which is generated is "hostname:filename", not just "filename". The resulting filename is invalid on Windows, and it is impossible to update the file as a result.