Fixing setting scrollLeft not firing callback

[KamranAsif]

Description

onScrollEnd/onScrollStart callbacks weren't firing when scrollLeft is set manually by the user.
_didScrollStop was being called, but _.isScrolling flag wasn't set. This is because _didScrollStart wasn't fired on initial call.

Motivation and Context

#53

How Has This Been Tested?

https://jsfiddle.net/nprp19me/

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.

[wcjordan]

lgtm