[MRG] No need to monkey-patch Click.HelpFormatter

[dimitern]

While working on adding tests for Excalibur (camelot-dev/excalibur#49), I've noticed this code:

def _write_usage(self, prog, args='', prefix='Usage: '):
    return self._write_usage('camelot', args, prefix=prefix)


# monkey patch click.HelpFormatter
HelpFormatter._write_usage = HelpFormatter.write_usage
HelpFormatter.write_usage = _write_usage

This monkey patching causes Excalibur's (or any other click-based) CLI to return camelot for the program name in the help output, which is a bit confusing, so I decided to propose this PR to fix it.

In order not to show camelot as the program name in the help output (passing --help), it's enough to set @group(name='camelot') on the main Click group for the CLI. The existing approach which monkey patches click.HelpFormatter.write_usage(), will affect any other Click CLI tool that uses camelot as its dependency, e.g. Excalibur.

Added a test to verify it works as expected in the help output.

[codecov-io]

Codecov Report

Merging #5 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master       #5   +/-   ##
=======================================
  Coverage   87.64%   87.64%           
=======================================
  Files          13       13           
  Lines        1505     1505           
  Branches      348      348           
=======================================
  Hits         1319     1319           
  Misses        129      129           
  Partials       57       57

Impacted Files	Coverage Δ
camelot/cli.py	86.32% <100%> (ø)	⬆️

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