Hover rating step configuration

## I'm submitting a...

[ ] Bug report  

[x] Feature request

## Current behavior

On hover the rating jumps one by one(1 star, 2 stars...)

## Expected behavior

i'd like to have them jump half by half(0.5 star, 1 star, 1.5 stars...)

## Environment

Library version: ^4.0.0-beta.3

Angular version: ^6.0.6

Start at the star-rating component API and the code handling hover input; inspect how the current whole-star increments are calculated. Use the example in the issue to verify that hover values can progress by half stars, and confirm the existing whole-star behavior remains available.

Written by the indexing model from the issue text.