Added a user tag to option structs

This can be used to further classify options.
Parser functions and option structs have been adapted accordingly.
Moved general options in separate function. Distinction of options by tag.
Split parse_flow_option into two separate functions.

parse_flow_option parses options without endpoint.
parse_flow_option_endpoint parses those that have an endpoint string.
[Code Style] Modified struct names in fg_argparser to comply with rest of project. Added doxygen to fg_argparser files.
[Code Style] Added doxygen comments
Merge branch 'controller_add_tag' into next
Merge pull request #115 from erzb5e6f65/next

Introduce a user tag in argparser option struct