Change to use Argument instead of Option on IndexCommand.Options.compilerargs

[norio-nomura]

Fix #480

[norio-nomura]

That is caused by illegal handling of spaces in compiler arguments.

$ echo 'print("space hello")'>space\ hello.swift
$ SOURCEKIT_LOGGING=1 sourcekitten index --file "space hello.swift" --compilerargs -- "`pwd`/space hello.swift"
sourcekit: [2:sourcekitd_initialize:4363: 0.0000] initializing
sourcekit: [2:sourcekitd_send_request_sync-before:4363: 0.0008] {
  key.request: source.request.indexsource,
  key.compilerargs: [
    "/Users/norio/github/swift-dev/SourceKitten/space",
    "hello.swift"
  ],
  key.sourcefile: "/Users/norio/github/swift-dev/SourceKitten/space hello.swift"
}
sourcekit: [2:sourcekitd_send_request_sync-after:4363: 0.0476] 
[1]    84434 illegal hardware instruction  SOURCEKIT_LOGGING=1 sourcekitten index --file "space hello.swift"  -- 

[jpsim]

Thanks for the fix!