Allow specifying wsconfig.json location

[Jand42]

Adding WebSharperConfigFile MSBuild property, automatically passed to WebSharper compiler to specify where to look for configuration file instead of using hard-coded wsconfig.json only. Can be set explicitly, or automatically set to wsconfig.ProjectName.json if file that exists then wsconfig.json if that file exists.

Example:

<WebSharperConfigFile>mywsconfig.json</WebSharperConfigFile>

[Jand42]

Followup: do not pass config file location to compiler args, parse it from outside task and pass final options to compiler for more transparency. See #1085