Add Feature: Partial omnisharp.json override

It is a great feature for custom omnisharp-server config added by @d12frosted at PR Use "omnisharp.json" from solution path as config file automatically.

However, omnisharp-server only uses omnisharp.json if existes and ignores its config.json. So it is not convenient to override a few configurations since omnisharp.json should contains all configurations.

It is better for omnisharp.json only contains different values of config.json for easier management.