Closes #115

Adds new print() argument sort which can take "time" (historical sorting, the default) and "alphabetical" values. The default can be changed with the profvis.sort global option. Sorting alphabetically produces the same sort of flamegraphs discussed in https://queue.acm.org/detail.cfm?id=2927301.

Edit: The print argument is now aggregate (takes a boolean) and the global option is profvis.aggregate (idem).

We might want to consider making this sorting the default as it makes it much more likely to merge the boxes within rows:

Ideally, would be togglable from the web UI. For convenience the sorting is implemented in R, but we could generate both data frames upfront.