Handle one-character identifier correctly.

before:
```
>> FS.lex '{{a}}'
RLTK::LexingError: Unable to match string with any of the given rules:
        from /home/ursm/.rbenv/versions/1.9.3-p362/lib/ruby/gems/1.9.1/gems/rltk-2.2.1/lib/rltk/lexer.rb:141:in `lex'
        from /home/ursm/src/FlavourSaver/lib/flavour_saver.rb:40:in `lex'
        from (irb):1
```

after:
```
>> FS.lex '{{a}}'
=> [EXPRST, IDENT(a), EXPRE, EOS]
```