Html builder refactor

[protolambda]

Hey, ESdoc has been of great use so far, but some of the html generation was not customizable enough for me. So I changed the builder system a bit:

• Each builder is optional now.
• Moved the html plugin class to HtmlBasePlugin, and left a proxy to it. It works the same as previously, but people can extend the Plugin class of esdoc-publish-html-plugin now to add their own builders.
• There is a set of default builders, behaving exactly like the old hardcoded set of builders.
• Each builder has the same exec function signature. Constructors can be different, but I kept the default set of builders consistent (builders are added by providing a creator-function, which wraps instantiation).
• Each builder can be configured with builder-specific options.
• There are also a global options, available to each builder. Useful for things like generating custom navigation.
• Split the DocBuilder class in two: a base builder class named Builder that's very general, and DocBuilder. The old DocBuilder class is way too large for my liking, it's better to layer functionality.

Features:

• For the manual builder: file patterns for the badge can now be configured.
• For general: user can overrule the header-link behavior, and specify their own custom links.
• The file-path for coverage reporting data can be configured now.

See the new esdoc-publish-html-plugin README.md for details about the configuration format.

[protolambda]

Also, for a future PR, I'm looking to improve the manual builder; the current builder flattens the manual structure and breaks all relative links in the markdown (it doesn't even change .md to .html). I'm thinking of creating a second manual-builder class that will implement this, and then we can compare/merge/replace the builders later.