Support ES6 tagged template literals

This patch adds support for GraphQL syntax in ES6 tagged template
literals. (As used by `graphql-tag`, for example.) By default, GraphQL
syntax is highlighted in template literals tagged with `gql` or
`graphql`. This can be configured via the `g:graphql_tag_names` array.

after/syntax/javascript/graphql.vim

@@ -0,0 +1,18 @@
+" Prologue; load in GraphQL syntax.
+if exists('b:current_syntax')
+  let s:current_syntax=b:current_syntax
+  unlet b:current_syntax
+endif
+syn include @GraphQLSyntax syntax/graphql.vim
+if exists('s:current_syntax')
+  let b:current_syntax=s:current_syntax
+endif
+
+syntax region graphqlTemplateString start=+`+ skip=+\\\(`\|$\)+ end=+`+ contains=@GraphQLSyntax,jsTemplateVar,jsSpecial extend
+exec 'syntax match graphqlTaggedTemplate +\%(' . join(g:graphql_tag_names, '\|') . '\)\%(`\)\@=+ nextgroup=graphqlTemplateString'
+
+hi def link graphqlTemplateString           jsTemplateString
+hi def link graphqlTaggedTemplate           jsTaggedTemplate
+
+syn cluster jsExpression add=graphqlTaggedTemplate
+syn cluster graphqlTaggedTemplate add=graphqlTemplateString

plugin/graphql.vim

@@ -0,0 +1,3 @@
+if (!exists('g:graphql_tag_names'))
+  let g:graphql_tag_names = ['gql', 'graphql']
+endif