Improvement & cleanup on PR #7 (#8)

1. PrettierLinter: Don't warn for already formatted files
2. ESLintLinter: Don't throw on exit code 1
3. ESLintLinter: Fix error on occasional missing source parameter
4. ESLintLinter: Add --fix & --cwd config options
5. ESLintLinter, PrettierLinter, PrettierESLintLinter: Cleanup & normalize getDefaultBinary & getInstallInstructions
6. ESLintLinter, PrettierLinter, PrettierESLintLinter: Improve global fallback

Thanks @kuba-orlik (https://github.com/pinterest/arcanist-linters/pull/7/)