Support Vietnamsese when generating header ID

Support Vietnamsese when generating header ID

If this option is `true` (and when `auto_ids` is true) ID values in
Vietnamese will be converted to a readable form (non-accent version
of the origin text).

This option should be used only if `auto_ids` is true.

Example: "Đây-là-ví-dụ" will generate new ID `day-la-vi-du` which is very
readable by Vietnamese. When this option is `false`, its ID should be
`y-l-v-d` which is unreadable (non-sense ID).

Default: true
Used by: HTML/Latex convertor