New markdown parsing tools. Labels decorator and extension parser.

This MR is a part of issue #87.

What's changed:

1. Added MarkdownDecorator interface for decorating markdown nodes with our custom delimiters to process them later.
2. Added ExtensionProcessor interface. You can override it to process your own type of Markdown extension. All processors are defined as arguments to GitlabExtensionsDelimiterProcessor
3. There are also CompositeMarkdownDecorator and CompositeVisitor which are convenient way to provide our decorators and visitors to parser.

To be able to parse nodes where first-only delimiter is existent, we are decorating each of such nodes with out custom project-specific extension syntax. Extension syntax looks like this: %%%%%${EXTENSION_TYPE}_${EXTENSION_ARGUMENTS}%%%%% For example: We have label ~single defined in markdown. We are making custom LabelDecorator that replaces such nodes to our custom extension:

%%%%%LABEL_SINGLE_single%%%%%

In this case out LABEL extension provides its own arguments string which have its own syntax:

SINGLE_single

where SINGLE is node type (nodes can be single-word, multi-word and id-based) and single is node name.

After decorating, markdown parser can detect our custom nodes using delimiters %%%%% and launches our GitlabExtensionsDelimiterProcessor

It is detecting type of extension based on substring before the first _ symbol and decides which extension processor to launch, and propagates arguments to it.

Custom extension processor converts current Text node to its own type.

After all, our custom nodes can be processed using Visitor.