Allow era tags (e.g. e-2) for browser support versions

[timkelty]

Example for setting to "2 versions back":

$browser-minimum-versions: (
"chrome": "e-2"
);

Spawned from: #1524 (comment)

[d4nyll]

Is the reasoning behind this so that your code will always support the latest 2 versions? If so, it might not work as intended.

From what I understand, the usage data for the *-usage-threshold variables are included with each Compass release, and not updated each time you run compile. Thus, I assume that if this is to be implemented, the version for that browser will also be calculated from the latest version as included with the release, and not the latest version when compile is ran.

Unless Compass can somehow detect the latest version of each browser at the time when compile is ran, I don't see a lot of benefits using (and the effort in implementing) e-2 as opposed to just specifying an explicit version.

[timkelty]

@d4nyll Yeah, I was suggesting that knowing that it would be tied to the Compass release.

I guess I was assuming (perhaps incorrectly) that if Compass is using caniuse data, they would update the data with frequent releases to Compass.