able to configure all blank lines for SignatureGenerator

This builds on my last pull request by allowing all the different blank lines generated by the SignatureGenerator to be configurable. It also allows a filter for modules. The existing behavior remains the same. These changes allow users of `FSharpVSPowerTools.Core.dll` to customize the formatting.

https://gist.github.com/ctaggart/0205da3f153cd20b099d#file-diff-fsx-L139-L154
```
let formatSymbol displayContext symbol (sg: FSharpEntitySig) =
    let filterer = { 
        SignatureGenerator.Filterer.NoFilters with  
            MemberOrFunctionOrValueFilter = fun m -> sg.MembersFunctionsAndValuesSigs.Contains m.XmlDocSig
            ModuleFilter = fun m -> m.XmlDocSig = sg.XmlDocSig
    }
    let blankLines = { 
        SignatureGenerator.BlankLines.None with 
            BeforeMemberOrFunctionOrValue = 1
            BeforePublicNestedEntities = 0
            AfterPublicNestedEntity = 0 
            BeforeMembersFunctionsAndValues = 0
            BeforeTypeHeaderOpenDeclaration = 0
            AfterTypeHeader = 0
            }
    SignatureGenerator.formatSymbol getXmlDocBySignature 4 displayContext [] symbol filterer blankLines
```

With this, I was able to generate a complete list of new additions to the public surface area of FSharp.Core for 3.1 and 4.0.
http://stackoverflow.com/questions/27363777/any-benefit-for-targeting-f-runtime-for-f-4-0-or-3-1-instead-of-3-0/27664089#27664089
ugh, hold on, I'll revert this change. Didn't mean to commit.