improve vscode marketplace appearance

[JakeDawkins]

resolves #816

🔼 (what the marketplace header looks like now)

🔼 (what the marketplace header would look like with these changes)

What I did:

• Updated marketplace readme to more-or-less match the proposed changes to the editor docs
• Changed the banner color for the extension to be blue
• Changed the icon to be white on blue instead of teal

TODO:

• Update CHANGELOG.md with your change (include reference to issue & this PR)

[trevor-scheer]

This list is another "thing" that would probably become dated as we change and add commands. Though we don't currently have a docs "source of truth" the way we do for config, so go with what you think is best here...I don't feel too strongly.

[mountainash]

@JakeDawkins - just wanted to point out a super small typo (but could be a big "copy & paste" timesaver)... missing a comma at https://github.com/apollographql/apollo-tooling/blame/master/packages/vscode-apollo/README.md#L84

I know I could fork, commit, PR, etc. but for a single , I thought it's less work for you (as an active committer) to work it into your next update.

[trevor-scheer]

Thanks @mountainash! We don't publish the extension often, but this fix will land the next time we do.
fc415b5