FEATURE: add a snap flag to enable snapping to plausible steps on inc & dec in TimeView

[JamesAlias]

Description

I'm referencing the original issue from another repo here: neos/neos-ui#2247 to see where the idea originated.

I added a feature behind a flag called snap which when enabled slightly alters the way the increase and decrease methods of TimeView work.
The flag is optional and completely backwards compatible so there are no breaking changes.

Motivation and Context

When the user uses the today button he sets the time to now which, for example, could be 00:12.
With step for minutes set to 5 the increase or decrease buttons on minutes it will step to 00:17 or 00:07 respectively.
With snap enabled it will snap to the next plausible value depending on step, e.g. 00:15 and 00:10 respectively.

Checklist

• I have not included any built dist files (us maintainers do that prior to a new release)
• I have added tests covering my changes
• All new and existing tests pass
• My changes required the documentation to be updated
• I have updated the documentation accordingly
• I have updated the TypeScript 1.8 type definitions accordingly
• I have updated the TypeScript 2.0+ type definitions accordingly

[jonnitto]

@YouCanBookMe is there a chance that you will merge this and make a release?