Diagnostics for all buffers

[prabirshrestha]

This is part of Diagnostics Revamp and vim-lsp 2020

This introduces a new flag to show diagnostics for all buffer using :LspDocumentDiagnostics --buffers=*. Documentation has more details.

• implement support for show all document diagnostics for all buffers and servers
• implement support for showing only the default buffer diagnostics
• update documentation
• respect disabled buffer
• add unit tests
• remove old code

Not in this PR but something for the future in case we would like to support it:

In the future we could have --ui=loclist or --ui=quickfix or even custom ui via lsp#register_ui to register curstom ui. For now this is out of scope since lot of ideas still need to be flushed out. But if folks want to use quickfix for all buffers we can easily implement --ui support for it without custom ui support.

One could then do :LspDocumentDiagnostics --ui=quickfix --servers=vim,efm --buffers=* --exclude-buffers=1,2