Basic ESI Tags Processing

To support ESI resolver in hst caching components:

Initial features:

• support esi comment block. e.g., <!--esi ... -->
• support esi include tag for internal component rendering urls. e.g., <esi:include src="..." />
• support esi comment tag. e.g., <esi:comment text="..." />
• support esi remove tag. e.g., <esi:remove>...</esi:remove>
• global hst configuration to turn on/off esi processing
• variables (e.g., <esi:include src="http://localhost:8080/test_$(HTTP_COOKIE {lang}

  ).html" />) won't be supported by this issue.

• vars element support

Unsupported features:

• external urls are not yet included. By this issue, only local component render urls will be supported.
• following tags won't be supported by this issue:

• choose | when | otherwise
• try | attempt | except