[FEA] Support "union" of benchmark.json files

[dillon-cullinan]

Is your feature request related to a problem? Please describe.
ASVDB should be able to take the results of multiple benchmarks.json files and create a "merged" benchmarks.json that contains all benchmark info.

Example Problem

1. Branch-0.14 stable release contains benchmark A, B, and C data
2. Branch-0.15 stable release contains benchmark B, and C data
3. Benchmark A data does not render in asv publish

benchmark.json will be based on most recent branch, causing us to not render the data of benchmark A. We should be able to merge these two benchmark jsons to make sure all data is represented.

Describe the solution you'd like
A new method that can parse a set of files. Best available on the command line with an argument to provide a filepath to stored .json files to merge.