Add support for tags with planned support

[Sub6Resources]

Feel free to submit a pull request adding one or many of the following (all will be added by version 1.0.0):

• audio (see Add support for <audio> tag. #127)
• bdi
• bdo
• source
• sub
• sup
• svg
• track
• video (see Add support for <video> tag. #126)
• wbr
• iframe (see Add support for <iframe> tag #128)

[Sub6Resources]

bdi and bdo added in 0.5.2 (f5081f2)

[Sub6Resources]

sup and sub added in 0.9.1

[Sub6Resources]

iframe is now on the list of elements to be supported!