TableNodeFormatter#format_table() is renamed #format_gfm_table(). And it is invoked from a new #format_table() with an additional parameter "gfm_conformant"
introduced TableNodeFormatter::NotConformantStyleError
added a test for a table with multi headers
added a test for a table with multicolumn cells
added a test for a table with multirow cells
modified TableNodeFormatter#check_conformance_with_gf_style() to cover multi-headers and row/colum spans
added TableNodeFormatter#deep_copy_tree()
added a new parameter "tree" to TableNodeFormatter#format_table()
convert tables into html if they are not conformant to GFM style.
defined DescNodeFormatter#visit()