Pypyr

With eternal thanks to Reskov 🎉 for next level super useful improved error handling! 🙇

- 139 - pypyr now saves all run-time errors to context under *runErrors*. This means subsequent steps can use error information from previous steps.
- 135 - error information now include the pipeline source yaml line+col numbers

- `pypyr.steps.glob` lets you get a list of paths from a glob, and combine different globs into a combined output result list. Issue 145
- with much thanks to Reskov 🎉 , resolves issue 143 - merge step now supports special tag directives

- new Retry decorator allows steps to retry automatically when step encounters an error. ref 130
- fix run_step AttributeError might have caught or hidden AttributeErrors in the step code itself, rather than just when the step module didn't contain a run_step function. ref 129
- while loop checks `stop` condition only at end of each iteration. previously if a stop condition evaluated True even before the loop started the loop wouldn't run at all. now the loop will run once. If you don't want the step to run, use the `skip: True` decorator instead.
- it's now easier to write infinite While loops with an explicit `stop: False` condition

* add string interpolation to _pypyr.steps.pype_ input.

- get the current datetime stamp into the pypyr context as a formattable string, where you can specify your own datetime formats using the standard python date time replacement tokens like `%Y-%m-%d` for _YYYY-MM-DD_.

use the new cwd input on _pypyr.steps.cmd_ and _pypyr.steps.shell_ to set the current working directory for the program/shell that you want to execute. Supports relative paths.