GeanyVC: Add support for a custom external diff viewer.

[yqu]

If "geany_external_diff" is found in the path, it is run as
the diff viewer. It can be a script or symbolic link to any
custom diff tool, easily modifiable by the user.
Otherwise the usual diff tools are tried as previously.

Also useful to add some command line arguments to the diff command.

[frlan]

Sorry. I'm not exactly sure what the goal behind the change is. Can you please go into more detail? Would be great.

[yqu]

Hi,
This change allows the user to use an external program of his choice to view the diff of the current file. In my case, I have a particular commercial program that I'd like to use instead of meld which is also installed on my machine.

This is one way to do it, another would be to have an environment variable override the builtin list of diff programs.

[b4n]

IMO a better implementation would be to allow the user to configure the external diff command in the plugin's configure dialog, it'd more easy to configure (and less hackish IMO). I think this shouldn't be too difficult to implement.

[yqu]

Yes, I will do that and update this pull request.

[frlan]

closing this pr until further feedback