added tick-by and tick-to methods

Hi,

I was using the library and I missed some methods to set the progress in a more customized way (just the regular tick is not flexible enough for some cases).

So to improve that I added `tick-by` and `tick-to` methods that allow for custom incremental and custom direct setting of the progress.

I added an example using the `tick-by` which a think is great for using with stuff like stream progress. I didn't added an example for `tick-to` but I actually have a real use case for it, if you wanna check see this issue on `clj-http`: https://github.com/dakrone/clj-http/issues/219

Thanks