Add complete support for internal links

[javiereguiluz]

easybook allows to link to any book section using the #id-of-the-section syntax. Some other tools, such as LeanPub, allows to define arbitrary id in the contents. This can be useful for example to link directly to a list of elements:

{#shopping-list}
  * Bananas
  * Cream
  * Pie Crust

Reference: https://leanpub.com/help/manual#some-crosslink-details