Update aptitude.md: purge command + better desc

* `aptitude install {{package}}` also installs dependencies.
* `aptitude remove {{package}}` also removes packages depending on `{{package}}`
* `aptitude purge {{package}}` removes packages and also removes config files

pages/linux/aptitude.md

@@ -6,7 +6,7 @@
 
 `aptitude update`
 
-- Install a new package:
+- Install a new package and its dependencies:
 
 `aptitude install {{package}}`
 
@@ -14,10 +14,14 @@
 
 `aptitude search {{package}}`
 
-- Remove a package:
+- Remove a package and all packages depending on it:
 
 `aptitude remove {{package}}`
 
+- Do an `aptitude remove {{package}}` and remove all config files:
+
+`aptitude purge {{package}}`
+
 - Upgrade installed packages to newest available versions:
 
 `aptitude upgrade`