Support user configurable compiler flags via YCM

[jerrymarino]

Add the ability to read in flags. Consider using the ycmd_extra_conf system which allows the user to programmatically generate flags and store them in YCM on requests.

[jerrymarino]

This one is resolved now in the latest revisions in YCMD: ycm-core/ycmd@67c04da

In short, the client can now read in compile_commands.json, so now, it's just a matter of generating them.

For Xcode: I wrote a toy implementation to output CompilationDatabases by using Xcode's internal frameworks here: https://github.com/jerrymarino/XcodeCompilationDatabase . I'll see if we can get comp dbs generated for swift in XCPretty: xcpretty/xcpretty#281