Exclude SSI Comments

[khalwat]

Exclude Nginx & Apache Server Side Include (SSI) comments from being stripped via minification, the same way IE directives are not stripped.

Both Nginx & Apache SSI directives begin with:

<!--#

So we can just look for the # and don’t strip the comment in that case.

Nginx SSI: http://nginx.org/en/docs/http/ngx_http_ssi_module.html

Apache SSI: https://httpd.apache.org/docs/2.4/howto/ssi.html

Implemented via PR: #670