Add release post for webR 0.3.1

[georgestagg]

To be posted after webR 0.3.0 hits npm and CDN.

• Indended to expand on some of the items in webR's NEWS.md file. The post is perhaps a little heavy on details aimed at JavaScript developers, but hopefully still interesting to the general audience.

• Some of the example output is hard-coded in the source, rather than generated by knitr, e.g. JS output and errors. There is some custom styling near the start of the post to tweak the visuals for this.

• Includes updates to the version of webR itself shipped on the website - used for interactive examples.

---

Updated links to hotfix 0.3.1 release.

[georgestagg]

Thanks again for the proof read and review!