Sort order for eras

[adamgundry]

With eras profiling it would be nice if there was an option to sort the bands by era, because I think it will make the graphs easier to follow. Presumably --sort name uses lexicographic order, but I guess adding a natural sort order would suffice?

[fendor]

Implemented in https://github.com/fendor/eventlog2html/tree/fendor/eras-label, will open the PR once #178 is merged