add behat test for #862
move utilities for validating user ids/logins to FetcherUser class
move utilities for validating plugin names to FetcherPlugin class
convert Fetcher to abstract class
extract FetcherTheme class
extract FetcherComment class
keep 'not found' message in Fetcher child classes

otherwise, might get a mismatch between the class and the message
extract FetcherPost class
Merge pull request #863 from wp-cli/fetchers

Standardize methods for validating positional arguments