refactor: Refactor memory_usage module to use module config. (#515)

Also addresses a number of bugs:
- the percent sign not displaying correctly on some terminal emulators, including kitty
- changing the symbol in the configuration file didn't do anything
- swap being shown even if the system didn't have any

src/configs/mod.rs

@@ -11,6 +11,7 @@ pub mod go;
 pub mod hostname;
 pub mod jobs;
 pub mod kubernetes;
+pub mod memory_usage;
 pub mod nodejs;
 pub mod package;
 pub mod python;