Assign TIMEOFF_TLS_{CERT,KEY} to vars to be reused
Assign TIMEOFF_TLS_{CERT,KEY} to vars to be reused

So that we don't have to keep calling `os.Getenv()`, create two new
variables, `tlsCert` and `tlsKey`. I've left these scoped to the
`main()` function (rather than intialising them using `var` in the
package scope) as they're not used elsewhere.

mattbostock/leavediary