Generated id values are not inique

[SAKryukov]

To reproduce, use the following input Markdown:

Header A

Header A

[ end of Markdown sample ]

It will generate two identical id values, which makes HTML invalid.

Reason: bugs in index.js. First of all, var used_headers = {} is placed inside the body of the function md.renderer.rules.heading_open, it makes the used header set empty on every heading, effectively turning off uniqueness mechanism. slugify also can be fixed.