Move "NgramIterator" into "ngram" subpackage.
Rename "NgramIterator" to "Iterator".
Just copy "bufio.ScanWords" as "ScanTokens".
Replace "isSpace" with "unicode.IsSpace".
Compile the regexp for scanning symbols.
Check whether the charactor is a symbol or not.
Cosmetic change:
Check wheter the first charactor is a symbol or not.
Merge branch 'feature/stupid-tokenizer'