Refactored ILineReader slightly

[GStefanowich]

Just like how I implemented Filters to automatically be found using Reflection, I changed implementations of ILineReader to be found using Reflection using the ExtensionTypesAttribute:

[ExtensionTypes(".odt")]
internal sealed class OpenDocumentXmlReader : CompressedXmlReader
{ /* Stuff */ }

[ExtensionTypes(".log", ".json", ".txt", ".sql", ".xml", ".sample", ".csv", ".tsv")]
internal sealed class PlainTextReader : ILineReader
{ /* Stuff */ }

[ExtensionTypes(".pdf")]
public sealed class PdfReader : ILineReader
{ /* Stuff */ }

Now line reader files are self-contained and modifying FileExtensionParsing.cs isn't necessary.

I also added documentation for creating Filters to the CONTRIBUTING.md file

I'll look into some of the Regex performance stuff when I have more time, but this was quick and just some cleanup stuff