The context should be set in a separate file other than the rc file

It would be great if the context, changed with `task context <>` would be written to a differnet file and not the main rc file. I think that some users like myself, are using version control to maintain their taskrc file and changing the context all the time marks the file as changed in this directory. A configuration value like `contextfile` would be great.

BTW I tried using an include statement to workaround this but what currently task does is just putting `context=<>` in the end of `~/.taskrc` which ovverides what is written in the included configuration file.

I'm using the stable 2.5.1 version right now and I tried 2.6.0 as well and it acts the same.