Add singer custom taps/targets to api

Closes: #232 (closed)

Add in custom taps/targets along with the existing meltano optimized plugins.

• update to build a list of plugins before splitting them out, this list now layers in custom taps and targets. Its important to note that for now since it was a little complex to blend them I decided that if there was a meltano optimized definition and a custom tap/target definition then I will just take the meltano one. My thought is that later we can combine the two files into a comprehensive single file.
• The json schemas for the api extractors/loaders had a few bugs that I fixed here to allow everything to pass
• I changed a few tap definitions that were using the default key in settings and I see that its allowed based on the schema but I think it does the same thing as value and its only referenced twice so maybe we should remove it. I updated the definition and removed from the schema for now. @tayloramurphy Is this used for something else or is it the same as value?