feat: allow arbitrary keymaps

[folke]

This PR allows to add arbitrary keymaps (see #8).

For example to setup goto keyboard shortcuts for lsp:

local keymap_goto = {
    name = "+goto",
    h = { "<cmd>lua require'lspsaga.provider'.lsp_finder()<CR>", "References" },
    d = { "<cmd>lua require'lspsaga.provider'.preview_definition()<CR>", "Peek Definition" },
    D = { "<Cmd>lua vim.lsp.buf.definition()<CR>", "Goto Definition" },
    s = { "<cmd>lua require('lspsaga.signaturehelp').signature_help()<CR>", "Signature Help" },
    i = { "<cmd>lua vim.lsp.buf.implementation()<CR>", "Goto Implementation" }
  }

wk.register_keymap("g", keymap_goto, { noremap = true, silent = true, bufnr = bufnr })

[AckslD]

Cool! Thanks for the contribution @folke :) I made some further changes based on your PR, see #10. Feel free to check them out and let me know if you agree.