fix search plugin ignoring diacritics

[t-fritsch]

allows searching for diacritics as asked in #3492

to test it :

1. npm start
2. add a slide in the demo.html file :

   <section>
       <h2>DIACRITICS SEARCH</h2>
       <p style="word-wrap: break-word">
           ÁĂẮẶẰẲẴǍÂẤẬẦẨẪÄẠÀẢĀĄÅǺÃÆǼBḄƁʚɞCĆČÇĈĊƆʗDĎḒḌƊḎDzDžĐÐDZDŽEÉĔĚÊẾỆỀỂỄËĖẸÈẺĒĘẼƐƏFƑ
           GǴĞǦĢĜĠḠʛHḪĤḤĦIÍĬǏÎÏİỊÌỈĪĮĨIJJĴKĶḲƘḴLĹȽĽĻḼḶḸḺĿLjŁLJMḾṀṂNŃŇŅṊṄṆǸƝṈNjÑNJOÓŎǑÔỐỘỒỔỖÖỌ
           ŐÒỎƠỚỢỜỞỠŌƟǪØǾÕŒɶPÞQRŔŘŖṘṚṜṞʁSŚŠŞŜȘṠṢẞTŤŢṰȚṬṮŦÞÐUÚŬǓÛÜǗǙǛǕỤŰÙỦƯỨỰỪỬỮŪŲ
           ŮŨVWẂŴẄẀʬXYÝŶŸẎỴỲƳỶȲỸZŹŽŻẒẔƵlaáăắặằẳẵǎâấậầẩẫäạàảāąåǻãæǽɑɐɒbḅɓßcćčçĉɕċdďḓḍɗḏđɖʤdzʣ
           ʥdžðeéĕěêếệềểễëėẹèẻēęẽʒǯʓɘɜɝəɚʚɞfƒſʩfiflʃʆʅɟʄgǵğǧģĝġɠḡɡɣhḫĥḥɦẖħɧɥʮʯųiíĭǐîïịìỉīįɨĩɩıijɟjǰĵʝȷɟʄkķḳƙḵĸʞlĺƚɬ
           ľļḽḷḹḻŀɫɭłƛɮljʪʫmḿṁṃɱɯɰnʼnńňņṋṅṇǹɲṉɳñnjŋŊoóŏǒôốộồổỗöọőòỏơớợờởỡōǫøǿõɛɔɵʘœpɸþqʠrŕřŗṙṛṝɾṟɼɽɿɹɻɺ
           sśšşŝșṡṣʂſʃʆßʅtťţṱțẗṭṯʈŧʨʧþðʦʇuʉúŭǔûüǘǚǜǖụűùủưứựừửữūųůũʊvʋʌwẃŵẅẁʍxyýŷÿẏỵỳƴỷȳỹʎzźžʑżẓẕ
           ƶIǂʭʔʡʖǁʕǃʢ
       </p>
   </section>

3. load presentation in browser http://localhost:8000/demo.html
4. using search plugin, search for any diacritic, it should navigate to this slide and highlight the right text

Please note that it may not work for every diactritics in every character set (only tested on latin diacritics).
Anyway maybe the discussion opened at #2331 can make the problem much simpler if we decide to search any character without altering the user input string.

fixes #3492

[hakimel]

Appreciate you taking the time to look into this @t-fritsch. I agree with your conclusion that we should allow searching for any character. I've addressed this in master via 339dc70 + 20d9eaf

[t-fritsch]

you're welcome ! thanks for the fix :)