Add httpie cli plugins in favor of the new cli namespace. (#1320)

* Add `httpie cli plugins` in favor of the new cli namespace.

* Separate each task to individual modules.

* Move httpie.manager.plugins to httpie.manager.tasks.plugins

Co-authored-by: Jakub Roztocil <jakub@roztocil.co>

docs/README.md

@@ -2347,7 +2347,7 @@ However, it is not recommended modifying the default behavior in a way that woul
         *) echo 'Other Error!' ;;
     esac
 fi
-```
+```
 
 ### Best practices
 
@@ -2425,7 +2425,7 @@ $ httpie cli export-args | jq '"Program: " + .spec.name + ", Version: " +  .vers
 
 ##### `httpie cli plugins list`
 
-List all installed plugins.
+List all installed plugins.
 
 ```bash
 $ httpie cli plugins list
@@ -2436,13 +2436,13 @@ plugin installations on every installation method.
 httpie_converter (1.0.0)
   httpie_iterm_converter (httpie.plugins.converter.v1)
   httpie_konsole_konverter (httpie.plugins.converter.v1)
-```
+```
 
-#### `httpie plugins upgrade`
+##### `httpie cli plugins upgrade`
 
 For upgrading already installed plugins, use `httpie plugins upgrade`.
 
-```bash
+```bash
 $ httpie cli plugins upgrade httpie-plugin
 ```
 
@@ -2450,12 +2450,12 @@ Successfully installed httpie-plugin-1.0.2
 
 Uninstall plugins from the isolated plugins directory. If the plugin is not installed
 through `httpie cli plugins install`, it won’t uninstall it.
-
+
 ```bash
 $ httpie cli plugins uninstall httpie-plugin
 ```
 
-## Meta
+## Meta
 
 ### Interface design
 
@@ -2465,21 +2465,21 @@ httpie_converter (1.0.0)
 For example, compare this HTTP request:
 
 ```http
-POST /post HTTP/1.1
+POST /post HTTP/1.1
 Host: pie.dev
 X-API-Key: 123
 User-Agent: Bacon/1.0
 Content-Type: application/x-www-form-urlencoded
-
+
 name=value&name2=value2
 ```
-
+
 with the HTTPie command that sends it:
 
-```bash
+```bash
 $ http -f POST pie.dev/post \
     X-API-Key:123 \
-    User-Agent:Bacon/1.0 \
+    User-Agent:Bacon/1.0 \
     name=value \
     name2=value2
 ```