Add Issue mailto link to vendor_detail.html

Add mailto link to vendor_detail.html to be used for contacting about
Vendor Issues. The link populates subject field with "Vendor Issue -
(Vendor Name)", with (Vendor Name) being the name of the restaurant on
the detail page.
Merge pull request #97 from cwimbrow/fix/issue-mailto-link

Add Issue mailto link to vendor_detail.html