Feature Request: Allow config to define filetypes

To allow running custom languages

Also possible cmd insertions (i.e ${TMP} in C's cmd?

Example:
```lua
opts = {
    file_types = {
        d = {
            cmd = "dub run --single",
            extensions = { "d" }
        },
        c = {
            cmd = "gcc ${TMP} -o ${TMP}_output; ./${TMP}_output",
            extensions = { "c", "h" }
        }
    }
}
```