Sort HTML attributes by key

[bep]

HTML attributes are backed by a Go map, and since Go 1 the iteration order has been random.

This is not a problem for the browsers, but is a problem when using changed content as a way of detecting which files to deploy to a server.

This commit fixes that by ordering by attribute key.

Fixes #37

[yosssi]

@bjornerik Thanks for your great work!

If possible, I'd like to change the type of htmlTag.attributes from the map of the strings to the slice of the custom attribute type which has the key and the value fields so that we could keep the order of the attributes on the template file and we could avoid the sort process which might lower the performance.

[bep]

Sure - no problem, the fix I provided was the first I thought of. Performance is important!

[yosssi]

For now, I'll merge this PR. A PR which changes the type of htmlTag.attributes from the map to the slice is welcomed!