refactoring of TableCellNode#parse_first_token()
the application of BlockParser#tagfy_link() is moved into BlockParser#add_leaf()
move #add_leaf() from BlockParser to BlockNode
move #tagfy_link() and #in_link_tag?() from BlockParser to BlockNode
added VerbatimNode#add_leaf()
refactoring of BlockNode#add_leaf() and BlockParser#add_verbatim_block()
moved from BlockParser#add_verbatim_block() to BlockNode#add_leaf(), a procedure for pushing into the stack a VerbatimNode when an input line matches LINE_PAT::VERBATIM_BEGIN
removed BlockParser#add_verbatim_block()
removed a parameter "verbatim_leaf" from (BlockNode|VerbatimBlock)#add_leaf()
refactoring of VerbatimNode#add_leaf()
refactoring of BlockNode#add_leaf()
added a test which passes a string as an argument of BlockParser#parse()
BlockParser#read_lines() and #parser() now accept a string as their argument