Allow report export in library

[nmattia]

Right now the hyperion reports from doAnalyze are only available in JSON form (doAnalyze performs the benchmarks, the analysis and the JSON reporting). This means that, currently, the two ways to access those reports from other Haskell code is to either (1) export the reports as JSON and parse them back or (2) copy the logic from doAnalyze. A better way would be to have a function doAnalyze similar to:

doAnalyze
  :: Config -- ^ Hyperion config.
  -> [Benchmark] -- ^ Benchmarks to be run.
  -> IO (HashMap BenchmarkId Report)
doAnalyze = ...

that the user could use directly.