Provide a way to specify limit as string

[fproulx-dfuse]

It'd be nice to specify the Limit via some string (like https://github.com/ulule/limiter does - which is nice, but I like the simplicity our your lib ) , that is very convenient when you want to specify the configuration in a command-line flag or config file. Ex. 5-S (5 per second), 100-M (100 per minute).

[logicwonder]

GCRA implementation needs to allow for customizing both the rate and burst. Simplification may limit customization.

[logicwonder]

GCRM implementation needs to allow for customizing both the rate and burst. Simplification may limit customization.