Include provided id on the generated object (#30)

* Include provided `id` on the generated object

This allows for referencing mermaid figures using the `pandoc-crossref` filter.

Example:

    ```{.mermaid caption="Example with id" #fig:example}
    sequenceDiagram
        Alice->>John: Hello John, how are you?
        John-->>Alice: Great!
    ```
    
    This is an example paragraph with a reference to @fig:example.

* Add README info about ID

* Fix typo

Author: kohenkatz

README.md

@@ -55,6 +55,16 @@ Admittedly, this is uglier than the earlier syntax on top - but that's how Pando
 
 It's also possible to override global defaults by using environment variables. The name for these environment variables are the same as the attributes prefixed with a `MERMAID_FILTER_` so that `width` would be `MERMAID_FILTER_WIDTH`.
 
+You can also specify an ID to be applied to the rendered image. This may be useful to use [`pandoc-crossref`](https://github.com/lierdakil/pandoc-crossref) or similar packages to reference your diagrams, for example:
+
+    ```{.mermaid #fig:example}
+    // Your diagram code here
+    ```
+    
+    This text has a reference @fig:example which is automatically inserted.
+
+(Note that `pandoc-crossref` will automatically find and use the `caption=` option. Also note that the order of applying the filters matters - you must apply `mermaid-filter` *before* `pandoc-crossref` so that `pandoc-crossref` can find the images.)
+
 JSON and CSS configuration
 ---------------------------
 

index.js

@@ -18,7 +18,8 @@ function mermaid(type, value, format, meta) {
     if (type != "CodeBlock") return null;
     var attrs = value[0],
         content = value[1];
-    var classes = attrs[1];
+    var id = attrs[0],
+        classes = attrs[1];
     var options = {
         width: process.env.MERMAID_FILTER_WIDTH || '500',
         format: process.env.MERMAID_FILTER_FORMAT || 'png',
@@ -99,7 +100,7 @@ function mermaid(type, value, format, meta) {
     return pandoc.Para(
         [
             pandoc.Image(
-                ['', [], []],
+                [id, [], []],
                 [pandoc.Str(options.caption)],
                 [newPath, fig]
             )