Pypyr

- Context now has a generator iter_formatted_strings
- fileformat step
- fileformatjson step
- fileformatyaml step
- filereplace step

- New fetchjson step
- New fetchyaml step
- README updates

- Add recursive string interpolation so can format iterable and nested iterable types
- get_formatted will now, instead of raising typerror on non-str, use the recursive function to parse types other than str

- KeyNotInContextError and KeyInContextHasNoValueError added. Refactor throughout to use new errs rather than KeyError.
- NB: henceforth, Context throws KeyNotInContextError, not KeyError, on __ missing __
- echoMe refactor to deal more sensibly with non-str types
- py.step had unreachable code, removed.
- new steps:
- tar
- contextclear
- contextclearall
- stepsrunner had unreachable code (steps can't re-assign context to None due to python call-by-obj param passing)
- bug fixes for format strings missing f""
- bug fix: complex steps exploded if didn't have the optional "in" context specified.
- 100% test coverage as of now

- envSet from pypyr.steps.env uses format strings. Breaking change - previously these took values directly from context based on key.
- add TRACE level logging for full traceback

- PipelineNotFoundError and PyModuleNotFoundError added
- Red colour on exception bug - reset to normal terminal colour after error print done so subsequent cmds not also in red
- unit tests