Add plugin feature

[kateinoigakukun]

You can create external plugins in IBLinterFile.swift

import IBLinter
import IBLinterKit

struct FooRule: Rule { /*...*/ }

let linter = IBLinter()
linter.run(externalRules: [FooRule.self])

• [x] IBLinterFile.swift executor
• [x] Build cache manager
• [x] Remote plugin
• [x] Edit IBLinterFile.swift in Xcode

How this works

Build libIBLinterKit.dylib and execute swift -L /usr/local/lib/iblinter -I /usr/local/lib/iblinter -lIBLinterKit ./IBLinterFile.swift in CLI iblinter

Like this https://github.com/danger/danger-swift