Add zoom icon

[andreadelrio]

We'll need this for the Canvas zoom feature. See this screenshot for reference:

[ryankeairns]

As it stands, this design for Canvas does not necessitate a separate Zoom out icon (minus), but we may consider adding that version to EUI in order to complete the pair.

How do we feel about adding this to the EUI set? should it be pulled into EuiIcon from an external source?

[cchaos]

Would these also replace the Maps zoom icons?

[ryankeairns]

They could, yes.