cmd: Implement httpie plugins interface (#1200)

docs/README.md

@@ -1880,6 +1880,11 @@ $ cat ~/.config/httpie/config.json
         3) echo 'Unexpected HTTP 3xx Redirection!' ;;
         4) echo 'HTTP 4xx Client Error!' ;;
         5) echo 'HTTP 5xx Server Error!' ;;
+        6) echo 'Exceeded --max-redirects=<n> redirects!' ;;
+        *) echo 'Other Error!' ;;
+    esac
+fi
+```
 
 ### Best practices
 
@@ -1919,6 +1924,70 @@ And since there’s neither data nor `EOF`, it will get stuck. So unless you’r
 > By default the plugins (and their missing dependencies) will be stored under the configuration directory,
 but this can be modified through `plugins_dir` variable on the config.
 
+#### `httpie plugins install`
+
+For installing plugins from [PyPI](https://pypi.org/) or from local paths, `httpie plugins install`
+can be used.
+
+```bash
+$ httpie plugins install httpie-plugin
+Installing httpie-plugin...
+Successfully installed httpie-plugin-1.0.2
+```
+
+> Tip: Generally HTTPie plugins start with `httpie-` prefix. Try searching for it on [PyPI](https://pypi.org/search/?q=httpie-)
+> to find out all plugins from the community.
+
+#### `httpie plugins list`
+
+List all installed plugins.
+
+```bash
+$ httpie plugins list
+httpie_plugin (1.0.2)
+  httpie_plugin (httpie.plugins.auth.v1)
+httpie_plugin_2 (1.0.6)
+  httpie_plugin_2 (httpie.plugins.auth.v1)
+httpie_converter (1.0.0)
+  httpie_iterm_converter (httpie.plugins.converter.v1)
+  httpie_konsole_konverter (httpie.plugins.converter.v1)
+```
+
+#### `httpie plugins uninstall`
+
+Uninstall plugins from the isolated plugins directory. If the plugin is not installed
+through `httpie plugins install`, it won't uninstall it.
+
+```bash
+$ httpie plugins uninstall httpie-plugin
+```
+
+## Meta
+
+### Interface design
+
+The syntax of the command arguments closely correspond to the actual HTTP requests sent over the wire.
+It has the advantage that it’s easy to remember and read.
+You can often translate an HTTP request to an HTTPie argument list just by inlining the request elements.
+For example, compare this HTTP request:
+
+```http
+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
+$ http -f POST pie.dev/post \
+    X-API-Key:123 \
+    User-Agent:Bacon/1.0 \
+    name=value \
     name2=value2
 ```