Added two exercises

[mikecoop]

Added the difference-of-squares and secret-handshake exercises.

I didn't add them to the config file because I don't know where in the order they should go since they are both fairly basic exercises.

[kytrinyx]

Cool!

The build is failing with the following output:

$ bin/configlet .
Evaluating .
-> config.json does not include [secret-handshake difference-of-squares].
The command "bin/configlet ." exited with 1.

Would you add these problems to the config.json in the root of the repository?

[mikecoop]

Does the order of the exercises matter? Should more simple ones be at the beginning or just put them at the end?

[kytrinyx]

Simpler ones should be toward the beginning... ish. It's not an exact science, as you can imagine.

[jwood803]

I don't see why these can't be added to the end. If I recall, that's what I've done. The difference-of-squares exercise could go more toward the beginning, but that's just my opinion.

[kytrinyx]

Ok, I have no opinion as to what is easier or harder than anything else. If there's nothing obvious, then just stick it on the end.

[bressain]

I've just been going in the order of issue #2 which would be to tack them onto the end:

difference-of-squares,
secret-handshake

[kytrinyx]

Thank you!