Fix for "--tag" command-line argument

Fix for "--tag" command-line argument.  "expand_requirements_specifiers" requires
"tag" keyword argument.
Comments for "--tag" command-line argument fix

Comments for "--tag" command-line argument fix.
Fix for "--tag" command-line argument

Fix for "--tag" command-line argument.  Added "ignore_missing_refs" kwarg to
"expand_requirements_specifiers" call.
Fix for "--tag" command-line argument

Fix for "--tag" command-line argument.  Added "ignore_missing_refs" kwarg to
"do_requirement_recursion" "expand_requirements_specifiers" call.
Cosmetic change to fix for "--tag" command-line argument

Cosmetic change to fix for "--tag" command-line argument.  Added
"ignore_missing_refs" kwarg to "do_requirement_recursion"
"expand_requirements_specifiers" call.
Merge pull request #98 from braincorp/tag_clarg_fix

Fix for "--tag" command-line argument