Store the comma character as part of each function parameter

If we do not need to use optional parameters we can press ‘C-d’ to
remove it from the function call.  But since the comma is outside of
the template it will remain after the deletion.  This leads to output
such as:

    filter_input($foo, $bar, , ,)

If we make the comma part of the template we get this:

    filter_input($foo, $bar   )

Removing the optional parameters leaves too much whitespace behind,
but this is still an improvement overall.
Remove leftover whitespace after deleting snippet parameters

Many PHP functions accept optional parameters and programmers will not
always need to use them.  In that situation the user can press ‘C-d’
to delete the parameter.  However, this leaves behind chunks of
whitespace that looks ugly.  This patch adds a hook that removes all
of that whitespace so that the snippet output looks natural.
README: Describe the ability to delete parameters

This patch also removes the note about the project’s instability since
it is now ready for real-world use.
Update the version number to 1.0.0 for the first stable release