Allow adding local bin folders as installed versions

[glennblock]

First as I've said many times, I love svm! This is great work @paulbouwer, we are lucky to have you!

Second, it would be awesome if I can install my local binaries folder where I build scriptcs during dev as a diff version.

This would allow me to easily jump between the downloaded binaries and my local source. So imagine

svm install -path c:/src/scriptcs/artifacts/release/bin -alias "dev"

Would now add my bin folder. So I could then do

svm use dev

then when I am done I can switch back, like when I am about to present live and don't want failures because I am pointing to the dev build (which happened this week) :-) or if I wanted to use prod builds for other purposes like bug verification.

One key thing is I would not copy the files, instead I would bind to the -path folder (or make that an option), this way when I rebuild, I automatically am pointing to the latest if svm is pointing to that path.

[paulbouwer]

Have updated svm install to work as follows:

svm install <version> <-f|-from> <path> [-s|-snapshot]

  Install scriptcs version from path <path> as version <version>. Path may be a local folder or a local NuGet 
  package. If Path is a local folder, then a soft link to the local folder is created. Use the -snapshot option
  to create a snapshot of the local folder instead of a soft link. 

  examples:
  > svm install mybuild-0.10.1 -f 'C:\scriptcs\artifacts\Release\bin'
  > svm install mybuild-0.10.1 -from '/tmp/scriptcs/artifacts/Release/bin' -snapshot
  > svm install 0.10.1 -from 'C:\Downloads\ScriptCs.0.10.1.nupkg'