Add section with demos in readme

[snuyanzin]

Currently there are at least three existing demos done with http://asciinema.org and published via twitter.
It would be useful to have links to them all together in readme

1. https://asciinema.org/a/212301
2. https://asciinema.org/a/214120
3. https://asciinema.org/a/EGUcLB1y4RBBdf1rxKsErmOoM?speed=2.5&theme=solarized-dark&cols=78&rows=18

Also there is at least one issue with PR and demo, and the set of demo could grow

[snuyanzin]

Added demos page in wiki with links to asciinema demo https://github.com/julianhyde/sqlline/wiki/Demos

[snuyanzin]

Merged as b3674f4