Include a setup() function for user configuration

[yochem]

Hi! I really like the colors from this plugin, but would like to tweak it a little bit according to my taste. I think this is something that other users might also want. I've modelled this functionality to work like https://github.com/ful1e5/onedark.nvim, which is a colorscheme I really liked to use.

most lua plugins have setup function, which takes a table with configuration as argument. This pattern is followed with this PR. Users can configure the same options as before, but now via the setup function. It is also possible to override colors defined in this theme as well as whole highlight groups.

I hope you like the changes I've made, and you are willing to include these into the repository.

Some more information:

User can configure the following settings:

• transparent bg (default false)
• italic_comments (default false)
• disable_nvimtree_bg (default false)

And override the following:

• colors (overriding the definitions in colors.lua)
• groups (overriding the group highlights as defined in theme.lua)

Other things this PR/commit changes:

• colors.lua now returns a table with the colors, instead of a function generate() that returns this table.
• utils.lua is removed, because it only had one function that was called once (in init.lua:set().
• Ran stylua

[yochem]

For backwards compatibility, it might be good to also check the vim.g.vscode_* variables. Will add support for it now.