Pypyr

- the cli now takes --name as default. instead of `pypyr --name mypipe` it's `pypyr mypipe`
- log level defaults to INFO
- signal interrupt handled (ctrl + c)
- stderr output red

- Add custom errors definitions to pypyr
- Not particularly in use in the core (yet), but the plug-ins are using them
- Version increase for new patch release

- `asserts_keys_have_values(self, keys, caller)` is now `assert_keys_have_values(self, caller, *keys)`
- add pypi version badge to readme

- Context is now a class with useful helpers, subclassed from Dict. Didn't go whole hog on a full-on MutableMapping implementation, since the goal is to add to Dict, not override anything particular to Dict.
get rid of the old "static" "helper" class validations that operated on context - these are now on Context itself
change throughout to use new Context methods
- The old 'context' package renamed 'parser'. Parser is more descriptive for what these things actually do (they parse stuff, they're not the context itself)
change throughout where code referenced the old namespace
- yamlfile in parser added
- echo now uses interpolated string
- minor "undocumented functionality" fix to avoid double error when context_parser fails
- add env step for set, get and unset env variables

- this release uses shippable to tag release and push tags to master.

Use built in python logging inheritance. Steps only need to use python `logging.getLogger()` henceforth.