Simplify GitStats workflow

[maciekbanas]

The idea is that e.g. pulling package usage (running pull_R_package_usage() function) results with the plotly View/output.

Not only table is saved in GitStats, but user has at once a glimpse at what he has pulled.

[maciekbanas]

For the time being user needs to... create_gitstats(), set_host(), pull_() and then get_() or get_stats() or plot().

It would be easier if e.g. user choose to get_*_stats() and if the data is not already pulled, GitStats would automatically pull and view the plot. There would be one function instead of 2, 3 steps.

The plot probably should be written inside GitStats, so it is then reachable.

[maciekbanas]

This for the discussion.
@epijim @Cotau @kalimu @marcinkowskak @mattsecrest

[maciekbanas]

Done in #374