Extract Renderable interfaces to module

[iachettifederico]

This PR is a Work in progress, since it's missing some tests and documentation.

• extracted all the functionality of SGML to Renderable module
• extracted all the functionality of HTML to HtmlRenderable module
• extracted all the functionality of SVG to SvgRenderable module

All the tests that were on the repository are passing

[joeldrapper]

This is a great start! I’d quite like to separate the SGML stuff from the renderable stuff where SGML is a specific implementation of renderable that buffers SGML tags, but other objects could implement the renderable interface just by implementing call to take a buffer, view context and block and mutate the buffer before returning it.

Then the render method could just check that the object includes Renderable before calling it. Does that make sense?

[joeldrapper]

These names might not be quite right, but here's the main parts I think we should have:

1. Renderable — a module that declares that you implement the interface where call takes a buffer, context and block and returns the (mutated) buffer;
2. SGML — a module that implements Renderable with a buffer and the ability to dynamically define tag methods for a specific SGML language;
3. HTMLRenderable a module that extends SGML with a specific set of tag;
4. SVGRenderable a module that extends SGML with a different set of tags;
5. HTML a class that includes HTMLRenderable; and
6. SVG a class that includes SVGRenderable.

It might even make sense to have a layer between 1 and 2 that provides the buffer, context, capture, etc. without any of the SGML element macros.

[iachettifederico]

3. `HTMLRenderable` a module that extends `SGML` with a specific set of tag;

4. `SVGRenderable` a module that extends `SGML` with a different set of tags;

For 3 and 4 I thought of 2 optiions:

1. HTMLRenderable (whichi I actually wrote)
2. HTML::Renderable

For both of those might be pros and cons. I think I like the second one best, but I'm absolutely flexible about this

[iachettifederico]

@joeldrapper also, I'm working on a Linux machine and don't have a Mac. Could you help me solve failing Mac specs?

[joeldrapper]

Hey @iachettifederico, sorry this ended up going stale. I’m going to open a new PR for this.