Support for numeric-prefixed versions

Previously, if you wanted to have numeric versions (e.g. 37months) you'd get a compilation error if you tried to use it with ERB.  This would exception:

<% if 37months? %>...<% end %>

Now, the Context allows you to prefix numeric numbers with an underscore in ERB.  You still define the version as 37months but in code you'd specify this:

<% if _37months? %>...<% end %>