Fix perf regression in the Tokenizer

After profiling the module, I see that a lot of time was lost in `_emitToken` and `_emitIfToken` method.
This is due to a useless concatenation that cost a lot since those methods are in the hot path.

My measure on my machine: 

Version 2.3.1 :
-> % node bench2.js
htmlparser2:  01.86 ms/el

Version 3.1.2 without the fix :
-> % node tests/bench.js 
htmlparser2:  04.50 ms/el

Version 3.1.2 with the fix :
-> % node tests/bench.js
htmlparser2:  01.75 ms/el