Introduce valueGetter and valueSetter options

[stefanoverna]

I need to apply a typewriter effect to an input placeholder.. this generalization is perfectly backward compatible and makes the task trivial. Tests are all green!

[coveralls]

Coverage remained the same at 100.0% when pulling 0c659bb on stefanoverna:master into 7cffb2b on yuanqing:master.

[coveralls]

Coverage remained the same at 100.0% when pulling 0c659bb on stefanoverna:master into 7cffb2b on yuanqing:master.

[coveralls]

Coverage remained the same at 100.0% when pulling a343e6e on stefanoverna:master into 7cffb2b on yuanqing:master.

[coveralls]

Coverage remained the same at 100.0% when pulling a343e6e on stefanoverna:master into 7cffb2b on yuanqing:master.

[zeke]

whoa @coveralls.

@stefanoverna this looks interesting. I'm not the owner or maintainer of this repo, but an example in the readme explaining how to use malarkey with an input placeholder would be helpful. :)

[yuanqing]

@stefanoverna Thanks for this; merging

I'll be making some changes before cutting a release:

1. Prefer getter and setter
2. Add some tests with the two new opts
3. Edit the README a bit

(Also, thanks @zeke!)