feat: v_request2() allow Filter and Relation with request.

[wehsLai]

feat:
New function v_request2() use XML node concepts, allow user to send Filter and Relation with request, here are some examples.
style:
I use styler::tidyverse_style() for whole package at once, uniform indent by two spaces.
doc:
I use devtools::document() and pkgdown::build_site() to generate package documents.
DESCRIPTION file add author, import purrr.

Functions using v_request2(), receiving json:
=== matches.R ===
v_get_volley_match_list
v_get_volley_match
=== players.R ===
v_get_volley_player_list
v_get_volley_player
=== pools.R ===
v_get_volley_pool_list
v_get_volley_pool
v_get_volley_pool_ranking
=== statistics.R ===
v_get_volley_statistic_list
=== teams.R ===
v_get_volley_team_list
v_get_volley_team
v_get_volley_team_ranking
=== tournaments.R ===
v_get_volley_tournament_list
v_get_volley_tournament
v_get_volley_tournament_ranking

[raymondben]

Hi @wehsLai, thanks so much for the PR! Looks great. Sorry it took a while to get to.
I've merged this PR into a new dev branch, there are a couple of things I'd like to change before we pull it into the master branch. Some functions have changed their input parameters, which will break existing code, so I'd like to allow those functions to accept EITHER the old parameters or the new (parent, children) ones. I have modified a couple of functions already, see #10 for details.