Clean up Invoke-Pester interface

Standard PowerShell cmdlets use singular nouns for their Parameters and
standardized set of parameter names like 'Path'. Changing Invoke-Pester
interface to reflect this. To keep backwards compatibility the changed
parameters are aliased.
Make output better structured

Moving the margin before the [+] in the test output. This makes for a
tree-like structure in which it is easier to see what tests are in
Describe and what are in Context