[API] Reduce request length by specifying the length of markup

[wrenger]

Certain languages allow a lot of markup (LaTeX, HTML, Markdown with comments...). Including this markup as text makes the request longer than they have to be. For LaTeX documents, markup texts usually take up 20-40% of the entire annotated text.

Is the markup text used anywhere?
If not, I would prefer a more space optimal solution. We could allow specifying the length of a markup part (instead of its content).

 {"annotation":[
  {"text": "A "},
- {"markup": "<b>"},
+ {"markup": 3},
  {"text": "test"},
- {"markup": "</b>"}
+ {"markup": 4}
 ]}

For backwards compatibility, the API just could allow both.