Autodocument Snippet options

[LucHermitte]

Given a snippet (and everything it includes),

• automatically extract all the options that can tune the snippet expansion ;
• Present these options for the end-user to see,
• and may be to edit (beware of toggable options).

Options could be extracted from:

• lh#option#get(),
• lh#ft#option#get(),
• get({scope}:,...) ({scope} =~ '[bg]')
• g:opt, b:opt
• other functions called (in that case, we may need to provide a way to register (likely manually) the options used by each function)