Implement convert and while + file-based editing

HELP.md

@@ -90,6 +90,11 @@ str %>
 
 ### I/O & Editor Control
 
+By default, `str` launches with an empty string as its initial subject (`''`).
+The command also accepts a file as its only parameter (e.g. `str my-file.txt`).
+If a file is provided, `str` will read from the file **and overwrite the file
+with the current subject when `exit` is called.**
+
 #### `exit`
 Stop `str` and exit. This can also be done with `^C`.
 
@@ -401,6 +406,11 @@ Replace the current subject with "Lorem ipsum..." placeholder text.
 Can generate individual words, lines, or paragraphs.
 Example: `lipsum 4 word` -> `lorem ipsum dolor sit`
 
+#### `convert <from> <to>`
+Assuming the current subject is a valid single data document, convert it `from` a lang `to` another lang.
+Supports: `php`|`json`
+Example: `{"a": 1}` -> `convert json php` -> `['a' => 1]`
+
 #### User-specific setup files
 The file `~/.str.rc` is automatically executed when `str` starts up.
 You can use this to define a user-specific environment.