Added normalize option

[saschazar21]

This fixes issue #48.
Added an option to 'normalize' tags and exchange special characters like äøßç to their ASCII equivalents aossc.

This makes use of the safeTag() function and therefore depends on the value added to the slug property in the options object. However, for the standard RFC 3986 mentioned in the README, it should be a good fit.

[jarodtaylor]

@hswolff this PR looks good to go, but before merging I wasn't sure if you had some guidelines (e.g. bumping version numbers, etc.)

[hswolff]

Typically I follow the rule of "what's easiest for the contributor, and what's easiest for the maintainer". i.e. I ask what is required of the contributor for the PR to complete the work described and to not introduce any regressions (in code, style, etc), and from there I'm happy to do any other book-keeping that needs doing, i.e. bumping version numbers. Sometimes to even speed things along I'll be ok with updating documentation myself, however in this case there's no need thanks to @saschazar21 :).

Did I give you npm publish access @jarodtaylor? Happy for you to take complete ownership of this package.

[gijswijs]

I agree with @jarodtaylor that this PR is good to go. This should result in a minor version bump. I don't have npm publish acces (yet) but I'm going to merge it anyway. Will check with @totocaster if I can get npm access.

[totocaster]

@gijswijs, sure DM me your npmjs.org username. It can't find you by gijswijs.