Several improvements.

[L3MON4D3]

This has multiple more-or-less interdependent improvements, hence me doing this in one PR:

• Snippet-filetype can be overridden via filetype-setting in context

  s({trig = "in lua", filetype = "lua"}, ...)

  will be added to lua even if it is in an add_snippets-call to another language.
  This may seem a bit weird at first, but is actually useful since it decouples
  the loaded filetype (language in package.json, "filetype.lua/snipmate")
  from the filetype of the added snippet. This allows doing something like
  grouping snippets by some property other than its filetype (for example all
  snippets for some framework together) which wasn't as nicely doable before
  (it was possible by splitting snippets by framework and filetype, but this is
  better).
  Also allows adding a single snippet to multiple filetypes via multisnippet.

• Some performance/functional improvements for loaders.from_vscode. It's a bit
  cleaner now, uses multisnippet for multi-filetype/multi-trigger snippets
  (scope/prefix can have multiple entries), and does not deepcopy snippets, but
  uses duplicate_addable/expandable, a kind of thin wrapper around the
  original snippet which allows it to be added to multiple filetypes, without
  invalidate removing all snippets (kind of complicated :/).

• Finally, support for loading .code-snippets standalone, without a package.json.

[L3MON4D3]

Alright, I think that should be it.
Anybody wanna look through these changes, now's the time :)