added ability for substitute aliases when mapping is on single line

This PR was submitted on the symfony/Yaml read-only repository and moved automatically to the main Symfony repository (closes symfony/Yaml#9).

At present parser only substitutes aliases when mapping is in a multiline.
When the alias is used in the mapping stored in a single line, alias substitution doesn't work.

So I modified Parser to fix it.
should be something like "where a << is used"