Extract library versions into ext properties

[peter-tackage]

Implementation of #33. Currently I've only done it for the library dependency versions. Should I do the rest too? I've also extracted the versions when they aren't used multiple times, it just seemed neater, although it does add some cognitive load for looking up the version & extra work when adding new libraries (you need to add the version into ext properties list, rather than just copy the suggested gradle dependency).

[peter-tackage]

I now wondering if the dictionary approach might be better. Like this - https://github.com/AdamLuisSean/AndroidCleanVIPER/blob/master/dependencies/dependencies.gradle

[tomaszpolanski]

I don't think it matters, both approached are almost identical