Add edition for future compatibility (#111)

I think it should be fine for downstream crates.

Cargo.toml

@@ -2,6 +2,7 @@
 name = "colored"
 description = "The most simple way to add colors in your terminal"
 version = "2.0.0"
+edition = "2021"
 authors = ["Thomas Wickham <[email protected]>"]
 license = "MPL-2.0"
 homepage = "https://github.com/mackwic/colored"