[MRG] Implements start from slide feature

[aditya95sriram]

Implements #98 and allows presentation to be started from any slide

For example,

present --start 3 examples/sample.md

starts the presentation from the 4th slide onwards

CLI option naming and whether to use 0-based or 1-based indexing is up to the maintainers.

[aditya95sriram]

As an addendum, I found that this feature really came in handy while prototyping/developing slides. For instance, if you're working on slide 10 and need to repeatedly make changes and check how it looks, it's really annoying to skip the first 9 slides every time.