Apostrophes not being stripped from heading anchors

[chrisjrob]

I would expect a heading of ## We're on fire to have an anchor name of #we-re-on-fire, but in fact it is #We%u2019re-on-fire. To add an extra layer of confusion - if you copy the link it comes up as #We%E2%80%99re-on-fire.

I have got used to the capitalisation of the first letter of the anchor (unusual in markdown), but I believe that any consecutive non-word characters should be replaced with a single hyphen.

[jackycute]

related to https://github.com/hackmdio/hackmd-io-issues/issues/27