Add support for flavor based version.txt

[jega-ms]

My Android project have multiple flavor. e.g , brand_1 , brand_2 and brand_3 i would like to maintain the separate version.txt file each flavor. Looks there is no support on the current version. Could you please add the support ?

[netzwerg]

Thank you for your interest in this plugin.

I currently don't intend to support more complex scenarios, because I would like to keep the plugin as generic/simple as possible. Feel free to fork the repo and implement the desired functionality yourself. If your changes turn out to be useful for others and still simple (i.e. not complicating non-flavored use cases), I am happy to look into a PR!