Add a custom variant name

**Is your feature request related to a problem? Please describe.**

I often find myself always tinkering my catpuccin color palettes, but at the same time I don't want to replace all of the default colors variant. It depends on the situation e.g. I am working on my low light room, I usually switch from mocha to frappe because frappe is more satisfying to work with. The problem is now I have my new color palettes for frappe, but I sometimes going back to my config to toggle them in order to get my default ones (again because I like it).

**Describe the solution you'd like**

I want a custom variant name to work with when i am focusing on work, rather then going back and forth to toggle my custom palettes. For example by simple calling `vim.cmd("colorscheme catppuccin-custom1")`,

**Describe alternatives you've considered**

I was thinking maybe you guys can provide some config similar to the following:
```lua
custom_variants = {
custom1 = {
merge_from = "frappe",
values = {
maroon = "#ea999c",
peach = "#ff9b5e",
},
},
custom2 = {
merge_from = "mocha",
values = {
maroon = "#ea999c",
peach = "#ff9b5e",
},
},
},
```

**Additional context**

This is my custom config override for frappe variant, I like the color contrast sometimes, but I don't want to override the default palette because I like all of the default variants palette.

Anyway, thanks for a good work!. I've been using catppuccin since I started to switch to neovim couple months ago.