feat: implement timer module (#118)

Implement a timer module that takes a commandline argument, the number of seconds the last job took to complete, and displays it if appropriate.

Alters shell initialization files to compute this number using date +%s where needed.

Adds a config section to configure minimum amount of time before timer is shown (default is 2s)

docs/config/README.md

@@ -109,6 +109,33 @@ command had an unsuccessful status code (non-zero).
 symbol = "❯"
 ```
 
+## Command Duration
+
+The `cmd_duration` module shows how long the last command took to execute.
+The module will be shown only if the command took longer than two seconds, or
+the `min_time` config value, if it exists.
+
+::: warning NOTE
+Command duration is currently not supported in `bash`. See
+[this issue](https://github.com/starship/starship/issues/124) for more details.
+:::
+
+### Options
+
+| Variable   | Default | Description                         |
+| ---------- | ------- | ----------------------------------- |
+| `min_time` | `2`     | Shortest duration to show time for. |
+| `disabled` | `false` | Disables the `cmd_duration` module. |
+
+### Example
+
+```toml
+# ~/.config/starship.toml
+
+[cmd_duration]
+min_time = 4
+```
+
 ## Directory
 
 The `directory` module shows the path to your current directory, truncated to
@@ -366,3 +393,4 @@ The module will be shown if any of the following conditions are met:
 [username]
 disabled = true
 ```
+