Fix for meminterval not using specified interval

Fix for meminterval not using specified interval

Hugo seems to ignore the meminterval I specify and always uses it's default of 100ms.
This seems to be because Hugo tries to take the meminterval from the command line
(an Int) and converts it to a String and passes it to time.ParseDuration. If you pass a
different meminterval (such as `1000` as above) it will fail (time.ParseDuration requires
some units) and use the default instead.

Changed `meminterval` to be a String and added better documentation for valid time units.

Resolves: #2325