Grunt impress command

Added command line option to grunt to start the presentation.

ie `grunt impress:avoidingconditionals` 
I was thinking of changing AvoidingConditionals.html to something like presentation.html and then could use 
```grunt impress AvoidingConditionals```
then we wouldn't need to maintain this list... let me know what you think after you have had a look at https://www.npmjs.org/package/impress-server