Add support for vim-vsnip

[Shatur]

Is your feature request related to a problem? Please describe.
The readme says that currently only UltiSnips and Neosnippet is supported for snippets and vim-vsnip can be used for LSP snippets expansion. But vim-vsnip can have user-defined snippets too with very convenient syntax. But snippets from it is not displayed. It would be nice to use a single plugin for LSP snippets and for user snippets.

Describe the solution you'd like
Could you add a support for vim-vsnip as for UltiSnips and Neosnippet?
Something like the ability to specify vim-vsnip in g:completion_enable_snippet.

[haorenW1025]

That's definitely possible. @hrsh7th is there an api that I can get all the snippets available for the current buffer?

[hrsh7th]

Currently, no. But it should be implemented. I will implement it.

[Shatur]

Should the integration exist in vim-vsnip-integ or here? I mean is I opened the issue in the right place?

[hrsh7th]

I've added vsnip#get_complete_items(bufnr) to vsnip.
hrsh7th/vim-vsnip@e5c8019#diff-29608c6ada9f413a1379505a0c7556faR131

[haorenW1025]

@Shatur95 It will be nice if you try #82 and gave me some feedback.

[haorenW1025]

closed by #82.