Take latitude/longitude as a CLI flag, among other goodies

This should close #1, and it also adds a CLI flag to specify a custom temporary directory to hold the cache file. This makes it possible to write deterministic bats tests!

I also:

1. ran `golint` and fixed the one complaint (`s/cache_file/cacheFile`)
2. created `exitWith` as the primary means of exiting the program early
3. added a README