Misc NParser additions

1. Add <inp2:m_IfDataExists>...</inp2:m_IfDataExists> tag, that will allow to check for data presence in given template part without creating a separate block or template for it.
2. Implement "html:..." element name processing for "m_RenderElement" tag. Could be used, with tags, that require multiple block names to be passed in, but some of the blocks are empty, then use "html:" instead of block name.