Correcting binary number literal (#133)

* Do not highlight "0B" prefix because it's incorrect syntax
  for Elixir.
* Add support for underscore separator.

Fixes #129

syntaxes/elixir.json

@@ -192,7 +192,7 @@
       "name": "variable.other.constant.elixir"
     },
     {
-      "match": "\\b(0[xX]\\h(?>_?\\h)*|\\d(?>_?\\d)*(\\.(?![^[:space:][:digit:]])(?>_?\\d)*)?([eE][-+]?\\d(?>_?\\d)*)?|0[bB][01]+|0o[0-7][_0-7]*[0-7]+)\\b",
+      "match": "\\b(0[xX]\\h(?>_?\\h)*|\\d(?>_?\\d)*(\\.(?![^[:space:][:digit:]])(?>_?\\d)*)?([eE][-+]?\\d(?>_?\\d)*)?|0b[01](?:[_01]*[01]+|[01]*)|0o[0-7][_0-7]*[0-7]+)\\b",
       "name": "constant.numeric.elixir"
     },
     {