Update terminal tag to instantiate a new shell

This way, each terminal tag has its own shell object to track its own
state.

js/pgsh.js

@@ -105,4 +105,3 @@ function pgsh() {
     return output
   }
 }
-var pgsh = new pgsh()

tags/terminal.tag

@@ -2,7 +2,8 @@
  * # Terminal Riot Tag
  *
  * Provides a pretend commandline interface, capable of displaying output from a
- * "Shell" javascript object.
+ * "Shell" javascript class. Multiple terminal tags can be used on a page,
+ * acting independently of one another.
  *
  * ## Defaults
  *
@@ -11,7 +12,7 @@
  *
  * ## Usage
  *
- *     <terminal shell='jsobject' welcome='text' prompt='text'></terminal>
+ *     <terminal shell='jsclass' welcome='text' prompt='text'></terminal>
  *
  *     <script src='riot+compiler.min.js'></script>
  *     <script src='he.js'></script>
@@ -24,23 +25,22 @@
  * - riot.js (http://riotjs.com/)
  * - he.js (https://github.com/mathiasbynens/he) for HTML Entity conversion.
  *
- * ## Making a Shell javascript object
+ * ## Making a Shell javascript class
  *
- * Shell objects must be defined before the tag is mounted. Shells can keep
+ * Shell class must be defined before the tag is mounted. Shells can keep
  * prompt character and welcome message settings, and must define a `process()`
  * function. Here is the structure of a minimumal shell that does nothing:
  *
- *     // Contents of shell.js
- *     function() shell() {
+ *     // Contents of myshell.js
+ *     function() myshell() {
  *        this.prompt = ''
  *        this.welcome = ''
  *        this.process = function(input) {
  *          return ''
  *        }
  *     }
- *     var myshell = new shell()
  *
- *     // Shell object used by Terminal Riot Tag
+ *     // Shell class used by Terminal Riot Tag in index.html
  *     <terminal shell='myshell'></terminal>
  *
  * ## Special Return Values
@@ -57,7 +57,7 @@
 
   // Take defaults from shell, otherwise take from `<terminal>` options.
   var shell = { 'process': function() { return ''; } }
-  this.shell = window[opts.shell] ? window[opts.shell] : shell
+  this.shell = window[opts.shell] ? new window[opts.shell] : shell
   this.welcome = this.shell.welcome ? this.shell.welcome : opts.welcome
   this.prompt = this.shell.prompt ? this.shell.prompt : opts.prompt