Add tooltip definition for tooltip.el

[jsmestad]

Background

tooltip.el defines background to be lightyellow which is really difficult to read. Other themes seem to set the tooltip value and then inherit that value into other tooltip faces like company-tooltip.

Changes

• Add tooltip definition based on what used to be in company-tooltip
• Have company-tooltip inherit from new tooltip definition.

[belak]

Makes sense, but could you please provide a before and after screenshot? Thanks!

[jsmestad]

@belak

Before version

This is the after version (oceanicnext)

[belak]

Perfect. Thanks!