Add support for shared timezone abbreviations

[QOAL]

Many timezone abbreviations are shared by several timezones.

An example being CST:

• Central Standard Time
• China Standard Time
• Cuba Standard Time

The extension should come with a good default for each instance of this. (It may be possible to detect if the user is in the non-default timezone and select their timezone instead) And there should be a UI for selecting a preference. (Which would be used by default) It may be a good idea to provide converted times using all the offsets in the tooltip, on hover.

[QOAL]

This is largely resolved with the release of 1.20 285ad1b The only bits missing are:

• Detecting the users local timezone and setting defaults based on it. This should be a separate issue created at a later date.
• Showing all of the times for the abbreviation in the tooltip; I think this isn't hugely desirable though.