Support for F# source generators

[granicz]

This ticket is for implementing a basic F# source generator scheme for WebSharper inside wsfsc.exe. The idea is simple: if the file list to be compiled includes non-F# files f_1 ... f_n, we perform the following steps before compilation:

For each filename f_i with file extension ext_i:

1. We attempt to locate a source generator function GEN, from the first assembly A filtered from the list of assemblies passed on the command line, where A contains a new WebSharper assembly-level attribute, say, [<WebSharper.FSharpSourceGenerator("{ext}">], marking the assembly to contain a source generator for a given {ext} file extension.
2. We call GEN on f_i to obtain a list of generated filenames fg_1 ... fg_m.
3. We replace f_i with fg_1 ... fg_m in the file list to be compiled.

Once all f_is are processed, we compute a new list of non-F# files from the updated input file list and repeat the above process (caching the ext->A mapping) until there are no more non-F# files to be compiled.

At this point, we resume normal compilation with the final source file list and all other command line arguments.