Add ITypeParser interface

[akamsteeg]

In v2.0.0 we introduced the ICommandLineArgumentsParser (#8) and IHelpWriter (#11) interfaces but for a TypeParser there's no interface. For consistency, we should add a 'ITypeParser` interface too, so users don't need get confused about sometimes having to extend an abstract class and sometimes implement an interface.

Proposal:

public interface ITypeParser
{
    /// <summary>
    /// Gets the <see cref="Type"/> this <see cref="TypeParser{T}"/> handles
    /// </summary>
    Type Type { get; }

    /// <summary>
    /// Tries to parse the specified value to the specified parse result
    /// </summary>
    /// <param name="value">
    /// The value to parse
    /// </param>
    /// <param name="parseResult">
    /// The parsed value
    /// </param>
    /// <returns>
    /// True if value was converted successfully; otherwise, false
    /// </returns>
    bool TryParse(string value, out object parseResult);
}

We must also update the configuration and all usages to support the interface instead of the current abstract base class TypeParser.