Added plugin archive source abstraction

[lightglitch]

Implements #31

[buildhive]

Decebal Suiu » pf4j #102 SUCCESS
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[decebals]

I was very busy today. Tomorrow I will evaluate this PR.

[lightglitch]

Take your time, is not urgent.

[decebals]

See #43. Are you ok with my changes?

[lightglitch]

It's only naming, right?
I didn't use Repository because could be confused with repository of https://github.com/decebals/pf4j-update but it's fine.

[decebals]

I didn't know that you know about pf4j-update 😄 In pf4j-update we have UpdateRepository.
I renamed PluginSource in PluginRepository, I moved FileFilter variable from DefaultPluginManager and PluginRepository methods in DefaultPluginRepository as constructor parameter. Also, I made PluginRepository pluggable in DefaultPluginManager with createPluginRepository method (you can override this method if you want).