Bring back `feline.add_theme()`

[mawkler]

This PR seems to remove feline.add_theme(). It doesn't mention why, and add_theme() is still in the helpdocs which makes me belive that it was removed by mistake by @famiu.

[freddiehaddad]

I'm not sure of the history behind removing add_theme I'll look info that. In the meantime, use_theme exists and works. Will that help with your particular issue?

Example:

local theme = {
    black = "",
    skyblue = "",
    cyan = "",
    green = "",
    oceanblue = "",
    magenta = "",
    orange = "",
    red = "",
    violet = "",
    white = "",
    yellow = "",
}
require("feline").use_theme(theme)

[freddiehaddad]

Looking at the PR, I think you're right. I created a new PR #9 which adds it back, but I'm going to make sure nothing breaks before merging it.