Make name=>key mapping optional, add -i option

Previously, knife-psearch required a user-supplied mapping of
attribute names to a key path, similar to what the underlying API
requires.  This commit makes that mapping optional by automatically
constructing a partial search request based on one of the following:

- Any `-a ATTR` flags passed on the command line,
- The `--id-only` option passed on the command line,
- A default set of attributes for nodes to mimic knife-search

Otherwise, it will fall back to doing a full search.
Add a warning when falling back to full search
Add README
Release version 0.9.0