Integrate YCMD JSON settings

[ChoppinBlockParty]

Add ycmd-settings-json-filepath - path to YCMD settings JSON file. YCMD accepts a json file where the server settings are set, including a one-time HMAC. The file is deleted after the server starts. As for now, emacs-ycmd was just creating a temporary file from some hard-coded/customizable values. Since those settings have to direct relation to emacs-ycmd, rather only to YCMD, it is good idea to keep it this way and just feed the file adding an HMAC value.

That change will read the JSON file, insert HMAC and pass it to YCMD. Please, test it, feedback is appreciated, I will mainly test it on C/C++ and Go.

That is supposed to solve #472, #479.

I copied ycmd/default_settings.json to my .emacs. and use it

ycmd-settings-json-filepath (concat user-emacs-directory "ycmd_default_setting.json")