Flowcharts support

In order to include flowchart.js diagram, add the following attribute to the page metadata section:

---
flowchart: true
---

You may then start embedding flowcharts.

Flowchart

To embed an example flowchart, add code block section labeled with flowchart as a language:

```flowchart
  st=>start: Start:>http://www.google.com[blank]
  e=>end:>http://www.google.com
  op1=>operation: My Operation
  sub1=>subroutine: My Subroutine
  cond=>condition: Yes
  or No?:>http://www.google.com
  io=>inputoutput: catch something...
  para=>parallel: parallel tasks

  st->op1->cond
  cond(yes)->io->e
  cond(no)->para
  para(path1, bottom)->sub1(right)->op1
  para(path2, top)->op1
```

  st=>start: Start:>http://www.google.com[blank]
  e=>end:>http://www.google.com
  op1=>operation: My Operation
  sub1=>subroutine: My Subroutine
  cond=>condition: Yes
  or No?:>http://www.google.com
  io=>inputoutput: catch something...
  para=>parallel: parallel tasks

  st->op1->cond
  cond(yes)->io->e
  cond(no)->para
  para(path1, bottom)->sub1(right)->op1
  para(path2, top)->op1