Add option to force command to use current file's directory for the working directory.

[markbirbeck]

Different types of commands need different locations for their working directory. For example, commands like make will usually operate against some top-level directory, which in Sublime might be indicated by an open folder or the directory where a project file is located.

However, other commands, such as git might want to operate within the same directory as the currently selected file.

The fixes to issue #17 make the 'root' behaviour the default (i.e., the behaviour that commands like make would need). This issue adds an option to the command that allows the 'root' behaviour to be overridden.