Add **kwargs to append_conf()

[funbringer]

This would allow us to do the following:

from testgres import get_new_node

node = testgres.get_new_node(port=5432).init().start()
node.append_conf(fsync=False, random_page_cost=1.5)

[funbringer]

Had to fix settings according to this page.

[codecov-io]

Codecov Report

Merging #48 into master will increase coverage by 0.14%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #48      +/-   ##
==========================================
+ Coverage   97.02%   97.17%   +0.14%     
==========================================
  Files          17       17              
  Lines        1547     1556       +9     
==========================================
+ Hits         1501     1512      +11     
+ Misses         46       44       -2

Impacted Files	Coverage Δ
testgres/consts.py	100% <100%> (ø)	⬆️
tests/test_simple.py	99.81% <100%> (+0.37%)	⬆️
testgres/node.py	97.05% <100%> (+0.05%)	⬆️
testgres/backup.py	96.96% <100%> (ø)	⬆️

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