refactor: add s:get_block()
add g:operator_furround_default_block
fixed motion=line
add operator_furround_latex
+  if s:get_val('operator_furround_latex', 1)
fixed s:get_block_latex(motion, str) " {{{
+function! s:get_block_xml(motion, str) " {{{
add test in t/xml.vim