Feature branch support in Postgres importer

[Gerrrr]

Current behavior

./poetryw run hunter regressions window1min_time_rocks_on_close --branch 3.5
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/Users/asorokoumov/Projects/hunter/hunter/main.py", line 516, in main
    script_main(conf)
  File "/Users/asorokoumov/Projects/hunter/hunter/main.py", line 666, in script_main
    regressions = hunter.regressions(
  File "/Users/asorokoumov/Projects/hunter/hunter/main.py", line 286, in regressions
    target_series = importer.fetch_data(test, selector).analyze(options=options)
  File "/Users/asorokoumov/Projects/hunter/hunter/importer.py", line 463, in fetch_data
    raise ValueError("Postgres tests don't support branching yet")
ValueError: Postgres tests don't support branching yet

Expected behavior

if there are no regressions:

INFO: Computing change points for test window1min_time_rocks_on_close...
window1min_time_rocks_on_close: OK
No regressions found!

If there are regressions:

INFO: Computing change points for test window1min_time_rocks_on_close...
window1min_time_rocks_on_close:
    process_cumulative_rate_stderr:     506. -->     940. ( +85.5%)
    process_cumulative_rate_diff: 2.88e+03 --> 5.11e+03 ( +77.5%)
Regressions in 1 test found