0.20.0

Backwards-incompatible changes:

• Update helptext command to return { Name = ""; Text = "" }. Fixes #35.
• project command response now has 'null' for OutputFile and
  TargetFramework if a value cannot be determined.

Other changes:

• FSharp.CompilerBinding removed, and used parts absorbed. Fixes #17.
• ScriptCheckerOptions fetched with no timeout, and also stores them.
  Fixes #18, #28.
• If a .fs file is not in a loaded project, produce an incomplete
  typecheck environment for it to give basic results.
• Update parsing of project options to include ProjectReferences. Fixes #39.
• Separate parsing of commands, main command loop, and formatting of
  response message into separate modules.