Allow configuration of timezone

**Is your feature request related to a problem? Please describe.**
Timezone mismatches between the user (contributor adding streak stats to their readme) and the server often leads to unintuitive representation of their streak.

**Describe the solution you'd like**
Allow the user to set a `timezone` query parameter to configure the timezone the server uses when calculating streaks.

**Describe alternatives you've considered**
Another approach would be to allow app deployer to set an environment variable to configure the timezone. This is less flexible, but should be easier to implement than the preferred method.