FR: support specifies the bufnrs for the buffer source

[gh-liu]

I sometimes want words from other buffers in a tabpage, after I read the code, I find it always get current buffer, could we add a optional func to specifies the bufnrs?

[gh-liu]

cmp-buffer support this.

[saghen]

Duplicate of #190

[gh-liu]

I am sorry to make this duplicate.