Introduced plugin architecture to allow for custom log file format editing & parsing #242

[Kittyfisto]

Fixes #242

Changes proposed in this pull request:

• [x] TextLogFile requires an ILogFileFormatMatcher to establish the log file's format
• [x] TextLogFile makes use of an ITextLogFileParserPlugin to parse both timestamps and log levels
• [x] Plugins may implement ITextLogFileParserPlugin and ITextLogFileParser to customize parsing attributes from the log file
• [x] TextLogFile will revise its log file format detection until 512 bytes have been read
• [x] Introduced LogFileProperties.FormatDetectionCertainty: It'll be None / Uncertain until the format is "locked" in and won't change anymore
• [x] Add Serilog plugin to tailviewer to allow parsing of custom (text) log file formats
• [x] Implement partial parsing of serilog format strings such as {Timestamp:yyyy-MM-dd HH:mm:ss.fff K} [{Level:u3}] {Message}
• [x] Allow users to specify their own custom formats in the UI

Any expected problems concerning backwards compatibility of existing plugins?
No

Any expected problems concerning backwards compatibility of existing user settings?
No

Does this break existing user workflows?
No