Initial implementation of wp media import

Introduces `wp media import` command to sideload images and import
them as attachments. Needs tests, second opinion, etc.
Add capability to download files from URL

If a recognizable scheme is passed in the <file> argument, `wp media
import` will attempt to download the file to a temp directory before
importing it.
Remove filename completion for wp media import

Filtering bash completions breaks bash's default filename globbing,
which is a more useful feature here.
Rename arguments to make more logical sense

Use the same field labels as are used by the admin media uploader, to
avoid confusion: title / caption / alt / description; rather than the
variable names used internally, where 'description' is really title and
so on.
Add Behat feature for `wp media import`
Add `--featured_image` associative arg

This flag will set the uploaded image to be the post thumbnail of the
post its attached to.

Also, fix the success message and the Behat feature, so that tests don't
fail on environments that use a different tmp path.
Update man page for `wp media import`

Add option description for `--featured_image`, and briefly explain examples
For local media imports, copy to tempfile before importing

Apparently `wp_handle_sideload` performs a`rename()` operation on the
file it's sideloading. In this case thats an unexpected side effect -
you don't want the original image to go away when importing it to
WordPress.
Add behat features to test against local files

One test for failure on trying to import non-existant image,
and one to make sure that local images are not removed on import
Use variables in feature steps for readability

Rewrote the feature step so that it displays the actual command being
run, and uses variable substitution.

* I had to modify the definition of the "file should exist" step to
  allow variable substitution. All of the existing tests still pass, but
  I'm not sure if this is best practice. I can write a new step instead,
  if that makes more sense.
Merge pull request #367 from goldenapples/goldenapples/media-import

Implement `wp media import` command