Allow path completion on notebook.

add -(dash) /(slash) \(antislash for windows) : (colon) ~(tilde)
as part of words symbol for notebook completer to help for path
completion

Fixes  #1969

IPython/frontend/html/notebook/static/js/completer.js

@@ -104,7 +104,7 b' var IPython = (function (IPython) {'
 
         // we need to check that we are still on a word boundary
         // because while typing the completer is still reinvoking itself
-        if (!/[0-9a-z._]/i.test(pre_cursor)) {
+        if (!/[0-9a-z._/\\:~-]/i.test(pre_cursor)) {
             this.close();
             return;
         }