adding requirements file
adding missing newline at EOF
improving resilience of default config

Due to possible KeyErrors it was not possible to set a default port but
not a default host. Using get() solves this problem. The try is still
needed in case the yaml is malformed, although silently ignoring it is
probably not the way to go.
adding support for non-root context

This fixes #7 and is a first step for
[ImmobilienScou24/yum-repo-server#44](https://github.com/ImmobilienScout24/yum-repo-server/issues/44)
displaying config file loading errors

instead of just silentlyi ignoring it
we are no longer required to set all configuration options
document DEFAULT_CONTEXT configuration
improve CLI interface by documenting the default context
Merge branch 'master' of https://github.com/ImmobilienScout24/yum-repo-client