Use Dependency Injection to allow output to be either Console or a string. This allows direct unit testing of SearchAndReplaceTokens & ScanAndPrintTokens.

Create IHandleOutput.cs for DI. Spelling corrections to README.md.
Add new tests for SearchAndReplaceTokens & ScanAndPrintTokens. These are high level test and may make some of the low lever test unnecessary.
The DI change means the third parameter on ScanForTokens is no longer needed.
Remove unnecessary _stopReplacements variable; revealed during unit testing.