Use toml + serde crates instead of custom config

[Bedis Nbiba]

A lot of Options logic could be simplified by using serde + toml,

I didn't use them at first because I cared about compilation time and reducing dependencies number, but now irust already uses some heavy dependencies like syntect so other crates like serde and toml (which are probably already used in the dependency tree) wont change much

[Bedis Nbiba]

Fixed in 8e43fdb