Fork updates

[voku]

• support for more unicode chars
• language-specific stop-words (disabled by default)
• currency support
• php7 types

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This change is 

[lux]

I just had a chance to benchmark how these changes would affect the library's performance. Calling ::filter() 100k times had the following results:

Version	Time
master before voku/portable-ascii merge	0.936257
master after voku/portable-ascii merge	5.796683
this branch	7.003144

I don't see this being a performance-critical piece of most apps, since once generated presumably the slugs would be stored somewhere, but I wanted to ask whether there were any options that can be tuned to speed things back up a bit, or if voku/portable-ascii provides any way to fall back to PHP extensions for parts of it when available?

[voku]

@lux I optimized the performance of the fork, if you have time, please check it. :)

-> https://github.com/voku/urlify

[lux]

Much faster! 1.768 from 5.7 or 7, nice :)

I see you closed the PR though. I didn't mean to imply I wasn't interested in merging your changes, just something I thought to look at to be thorough. Any chance of getting the performance improvements you made into this PR still? That would be awesome.

[voku]

@lux I back ported the changes, now we will easily call \voku\helper\ASCII::to_ascii() :)

[lux]

Thanks, much appreciated!