buffer: currenttime

[andreyrd]

What kind of change does this PR introduce? (check at least one)

• Bugfix (non-breaking change which fixes an issue)
• Feature (non-breaking change which adds functionality)
• Code style update
• Refactor (refactoring or adding test which isn't a fix or add a feature)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

Did you test your solution?

• I lightly tested it in one browser
• I deeply tested it in several browsers
• I wrote tests around it (unit tests, integration tests, E2E tests)

Using this in production, tested in Safari, Chrome, and Firefox.

Problem Description

Currently, web-audio-player does not implement .currentTime for buffer sources.

Solution Description

This implements that, even if the player is paused and started again.

Side Effects, Risks, Impact

None that I know of.

• N/A

Aditional comments:
This is different from #7 in that it properly handles a paused and restarted player.