Ignore nested element start tags

This ensures that Oga is able to tokenize input such as the following:

    <script<script>foo</script>

Oga will now treat this as:

    <script>foo</script>

This is based on libxml behaviour, which seems to differ a bit from
Chromium which treats the node as a text node. This however would
require complex look-ahead logic (as far as I can tell) that I really
don't want to implement in Oga.

Fixes #186

ext/ragel/base_lexer.rl

@@ -389,6 +389,7 @@
 
     element_start = '<' ident_char;
     element_end   = '</';
+    element_start_pattern = '<' identifier (':' identifier)?;
 
     # Machine used for lexing the name/namespace of an element.
     element_name := |*
@@ -551,6 +552,7 @@
     # Machine used for processing the contents of an XML element's starting tag.
     element_head := |*
         newline => advance_newline;
+        element_start_pattern;
 
         # Attribute names and namespaces.
         identifier ':' => {
@@ -578,6 +580,7 @@
     # tag.
     html_element_head := |*
         newline => advance_newline;
+        element_start_pattern;
 
         html_identifier => {
             callback(id_on_attribute, data, encoding, ts, te);