Ycecream

=========================
There was still a __all__ definition left over, which caused a problem when using ycecream from site-packages
(installed with pip).
Bug fixed.

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It is now possible to use the string 'stderr', 'stdout' and 'null' as output attributes.

Ycecream will now try and read ycecream.json (in the directories in sys.path) to override
the default values, e.g.
{
"prefix": "==> ",
"show_time": true,
"line_length": 120
}

Internal change: all defaults are now stored in a class defaults.
Internal change: phased out stderr_print.

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The y decorator now also adds the duration of the function/method call on exit, like
y| returned None from wait(5) in 5.004965 seconds
This can be useful as a basic way to benchmark a function.

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y can now be used as a decorator to debug (keyword) argumnets given to a function.
In connection with this there are two new parameters:
- show_enter
- show_exit

Changed the name of several parameters:
- show_context (was include_context)
- show_time (was include_time)
- show_delta (was include_delta)

The parameter output_function is now called output. And it can now be:
- callable (like before, e.g. print)
- an open text file (e.g. sys.stdout or an explicitely opened file)
- a str or Path object that will be used as the filename (opened with 'a+' for each call)

The parameter arg_to_string is now called serialize.

Complete rewrite of tests, now in PyTest.

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Phased out Y.format() and replaced by Y(as_str=True)

Major update of readme.md

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Several functionality changes