Added option to only show available packages

Added the --available (-a) option to the show command to only list
the available packages, similar to the --installed and --platform
options.

Additionally changed the output formatting when limiting the
package result to remove the hierarchy when only one type is being
showed. This facilitates parsing of a list of packages (for example
for shell scripting and completion).
Added option to only show package names

Added the --name-only (-N) option to the show command to only list
package names (and exclude version and description).

This is useful to produce a list of package names to be parsed by
a shell script for example (bash completion comes to mind).
Merge remote-tracking branch 'gerryvdm/master'

Conflicts:
	src/Composer/Command/ShowCommand.php
Show proper repo information and not always the default ones