Allow `wp post create` to read from file or STDIN

If a filename is passed as the first argument to `wp post create`, that
file will be read for the post's content. If the first argument is '-',
then post content will be read from STDIN.

If an unreadable filename is given, `wp post create` exits with an
error.
Update man page for `wp post create`

Still can't get synopsis to render correctly though. After adding
optional positional argument to synopsis, all calls generate an 'unknown
parameter' warning.
Add filename completion for `wp post create` command

Uses bash default filename completion for `wp post create`