read factory function from Args class

[vanyakosmos]

• [x] rename constructor to factory
• [x] read factory function from Args class by string
• [x] ... from default name (eg def read_OPTION(value: str)...)
• [x] allow to parse args based on instance of Args class and populate self in factory methods
• [x] docs

[codecov[bot]]

Codecov Report

Merging #17 into master will not change coverage. The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #17   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           8      9    +1     
  Lines         637    661   +24     
=====================================
+ Hits          637    661   +24

Impacted Files	Coverage Δ
argser/consts.py	100% <100%> (ø)	:arrow_up:
argser/fields.py	100% <100%> (ø)	:arrow_up:
argser/utils.py	100% <100%> (ø)	:arrow_up:
argser/exceptions.py	100% <100%> (ø)
argser/parser.py	100% <100%> (ø)	:arrow_up:
argser/display.py	100% <100%> (ø)	:arrow_up:

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