a new type of strings is supported inspired by the ruby strings.
%<delimiter>content<delimiter>, if opening delimiter is one of
([{<, then closing delimiter is the matching )]}> and balanced
delimiters in the string needs not to be escaped, else the closing
delimiter is the same as the opening one.
shell expansion is available through %sh<delimiter>command<delimiter>
syntax.
Command flags have been removed, as these strings provide proper nesting
support, so now, you can for example do:
def command %{ echo %sh{ ls } }
