Add argument completion for `:ColorSwatchGenerate`

[obcat]

Hi,

I added support for argument completion for the :ColorSwatchGenerate command.

• 1st argument

  :ColorSwatchGenerate <C-a>
  

  :ColorSwatchGenerate all original cterm
                       ^----------------^
  

• 2nd argument

  :ColorSwatchGenerate all <C-a>
  

  :ColorSwatchGenerate all default minimal csv css json
                           ^--------------------------^
  

I believe this could be helpful for those who don't know/remember what can be an argument. Please feel free to let me know if any improvements can be made.

Thanks!