oll-doc: Parse definition file (first step)

parse version, header section and snippet body into fields.

The parsing isn't very robust and relies on a certain level of
correctness in the input files.
oll-doc: Safety check for filecontent.
oll-doc: Parse header fields of snippets (WIP)

use a dictionary SnippetDefinition.headerFields
Populate this when parsing the headercontent.

For now it only understands fields with one quoted string in a line.
Others will return None.

Another step to be done: initialize mandatory fields with the dict,
so one can walk through that dict and determine missing fields
in the file.
oll-doc: Parse multi-line header fields too

ATTENTION: This is quite fragile!
To work the snippet has to be well-formed, particularly:
- the multi-line field has to end with a single '}'
  on a line.
- this line has to be indented (otherwise it would be
  interpreted as the end of the \header block
oll-doc: Predefine expected header fields