Add "Parse and increment a version string" example

[convoliution]

#283

I'm actually not certain if this follows best practices or even the intended goal of the example.
Feedback appreciated!

[convoliution]

Potential points to note:

• Changed name of example from "modify" to "increment" because incrementing is the only modification the crate enables.
• Prematurely assigned the semver crate to the Configuration category; the crate is not currently in any category, though I have mentioned this on the crate's evaluation thread!
• Not entirely sure if I'm implementing error-chain correctly...

[budziq]

That's excellent work @mykalu !
I've merged it after minimal cleanup (squashed commits into single one, small wording update and moved the the external reference link to the bottom of the file)