sprunge: add README and refactor (#8239)

Co-authored-by: Marc Cornellà <marc.cornella@live.com>

plugins/sprunge/README.md

@@ -0,0 +1,31 @@
+# Sprunge plugin
+
+This plugin uploads data and fetch URL from the pastebin http://sprunge.us
+
+To enable it, add 'sprunge' to your plugins:
+```
+plugins=(... sprunge)
+```
+
+## Usage
+
+| Command                      | Description                               |
+|------------------------------|-------------------------------------------|
+| `sprunge filename.txt`       | Uploads filename.txt                      |
+| `sprunge "this is a string"` | Uploads plain text                        |
+| `sprunge < filename.txt`     | Redirects filename.txt content to sprunge |
+| `echo data \| sprunge`       | Any piped data will be uploaded           |
+
+Once sprunge has processed the input it will give you a unique HTTP address:
+```
+$ sprunge "hello"
+http://sprunge.us/XxjnKz
+```
+
+## Notes
+
+- Sprunge accepts piped data, stdin redirection, text strings as input or filenames.
+  Only one of these can be used at a time.
+- Argument precedence goes as follows: stdin > piped input > text strings.
+- If a filename is mispelled or doesn't have the necessary path description, it will NOT
+  generate an error, but instead treat it as a text string.