Rework plugin structure to replace pluggable with a xml file

[hlxid]

Implements everything discussed in #41.

Todos:

• Implement PluginLoader to load plugins with the new plugin.xml in every jar. Reach metadata parity with pluggable.
• Remove pluggable from the api (Remove pluggable because its not used any more after plugin structure rework chatoverflow-api#11)
• Add new optional properties to PluginType that pluggable didn't had like version and website
• Add plugin.xml files to the official plugin repo (Replace pluggable classes with the new plugin.xml format chatoverflow-plugins#1)
• Enhance sbt create task (Closes Enhance plugin create task #20)
  • to autogenerate the plugin.xml
  • to autogenerate the class implementing PluginImpl in java/scala.
• Rework sbt fetch task to only fetch plugins with a existing plugin.xml file
• Add new properties to rest api
• Update the wiki to reflect these changes (Add explanation of the new plugin.xml file and add a migration guide chatoverflow-wiki#3)

[hlxid]

@sebinside this is now ready to be reviewed 😃

[sebinside]

Wow, this is huge - thank you! Although I followed the development progress... this might take some while :)

[hlxid]

No worries, it doesn't need to done within a certain time. Do it at your pace. :)

[sebinside]

Everything is fine, will be merged soon.

[sebinside]

Update npm package to version 0.3.4 with the new plugin metadata. https://www.npmjs.com/package/chatoverflow-api