Add "cxxstd" json field:

close #477

The "cxxstd" json field is being added to each Boost library's meta
json information for libraries whose minumum C++ standard compilation
level is C++11 on up. The value of this field matches one of the values
for 'cxxstd' in Boost.Build. The purpose of doing this is to provide
information for the Boost website documentation for each library which
will specify the minimum C++ standard compilation that an end-user
must employ in order to use the particular library. This will aid
end-users who want to know if they can successfully use a Boost
library based on their C++ compiler's  compilation level, without
having to search the library's documentation to find this out.

meta/libraries.json

@@ -13,5 +13,6 @@
     ],
     "maintainers": [
         "Vinnie Falco <vinnie.falco@gmail.com>"
-    ]
+    ],
+    "cxxstd": "11"
 }