add support for custom theming in mers errors

this also includes support for the NoTheme,
a theme which doesn't add any color to mers'
output.
If you compile mers with --no-default-features,
the `colored` dependency will disappear and
mers_lib will fall back to NoTheme.

mers_lib/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "mers_lib"
-version = "0.8.13"
+version = "0.8.14"
 edition = "2021"
 license = "MIT OR Apache-2.0"
 description = "library to use the mers language in other projects"
@@ -10,9 +10,14 @@ repository = "https://github.com/Dummi26/mers"
 
 [features]
 default = ["parse"]
+
+# for parsing and running mers code (for most situations: just enable parse)
 parse = ["run"]
 run = []
 
+# color features are used when formatting error messages.
+ecolor-term = ["dep:colored"]
+
 [dependencies]
-colored = "2.0.4"
+colored = { version = "2.0.4", optional = true }
 line-span = "0.1.5"