Add ability to import files from tsconfig

[dsherret]

ast.addSourceFilesFromTsConfig(tsconfigPath?: string)

• By default, it would use the source file specified when creating an ast.
• This would be opt-in, as shown.

[dsherret]

I believe ts.parseJsonConfigFileContent would be used for this. Might be better to make this an option like so...

const ast = new Ast({
    tsConfigFilePath: "tsconfig.json",
    addFilesFromTsConfig: true
});

...with it defaulting to true if tsConfigFilePath != null

[dsherret]

I don't believe it's worth it to work on this until ts.matchFiles is exposed in the compiler api. See here: microsoft/TypeScript#13793