Add user agent

[LMBishop]

Closes #7

[FrozenDude101]

Would it be better to place the User-Agent in the environment, rather than in the public code?
It contains an email, so CSS might get more spam.

[RogerGabeller-ml]

I did think of this but, the email's is publicly available anyway so it shouldn't matter if it can be found through this repository.

[LMBishop]

Would it be better to place the User-Agent in the environment, rather than in the public code?

They want the email of the maintainer of the code, not the actual user running the code, so it doesn't make sense for the email address to be variable. (Also we get a lot of random shit in the CSS inbox anyway because the email is already on the /about page.)