Add section name translate support
Add section name translate support

When section name is not written in ASCII, the ID of the section will be
generated by default, in the form of 'id<unique number>'. This is no good
writing links to other document, whose ID are subject to change in adding
or removing sections.

Adding a `translate_section_name` filter in Parser will allow user to install
a customized one to translate the section name into more readable one. For
example, convert the Chinese section name to more ASCII-friendly pinyin.

rtfd/recommonmark