Add concept of Syntax Registry

Right now we support well-known syntaxes which are hard-coded into the Syntax and SyntaxType classes but it's not possible to register new Syntax or Syntax Types.

This is a problem since it means it's not possible to have an extension (like the contrib Markdown one) contributing a new syntax (in the case of Markdown it's been hardcoded as a well-known syntax) or change the syntax pretty name. Right now the pretty name for "markdown" is "Markdown" but we need to change it (see MARKDOWN-72). We also need to introduce a new syntax, see https://jira.xwiki.org/browse/MARKDOWN-69.