Allow specifying pre-processor directives, global settings, etc. in docs recipe

[daveaglick]

Compilation customization could be improved, particularly when processing docs from source files. Some ideas:

• Specify pre-processor directives to pass to Roslyn when compiling from source files
• Specify additional global properties and environment variables for MSBuild when compiling from project/solution files

[SilentSin]

This sounds like a good idea. I'm generating documentation for a Unity project which includes unit tests, performance tests, and other experiments that I don't actually ship with the product. In theory I can just make a dummy project in VS that references the folders I want for Wyam to use (and I'll try that when I get home), but being able to just add a line in the config file would be nice.

Maybe something along the lines of Settings[DocsKeys.CompilationSymbols] = "UNITY_EDITOR;DEBUG;ETC";