Implement show methods

[Ellipse0934]

Currently highlight only supports html & latex MIME types which are written to a stream. For other julia projects to use Highlights it would be helpful to implement a show(x::Highlights.Compiler.Context) so it can be used easily by various outputs like Pluto(HTML), Terminal,.etc

[MichaelHatherly]

Yeah, adding shows for the valid MIMEs is fine by me.