Add ability to customize params per request about vim-lsc HOT 4 CLOSED

As an FYI after #64 this can have a bit nicer syntax:

 let g:lsc_server_commands = {
     \ 'c': {
     \    'command': 'clangd',
     \    'message_hooks': {
     \        'textDocument/didOpen': {'metadata': {'extraFlags': ['-Wall']}},
     \    },
     \  },
     \}

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Might be good to implement this on top of #39 rather than add another global variable.

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@hia3 - please let me know if this approach works for you.

See :help lsc-server-message_hooks for documentation and an example of how to set this up.

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@natebosch It works like a charm, thank you!

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