fix some highlight string colors

redbaron has some trouble highlighting properly strings containing quotes (')

Mainly visible with .help() functions, not sure default repr is affected.


Example with a foo.py file, trying to have most common quotable features:

```
def foo():
  a = 'Simple quoted \' string'
  b = "Double quoted \" string"
  c = '''Multi-simple quoted ' string'''
  d = """Multi-double quoted " string"""
def bar():
  e = 'Simple quoted \' string and some " for fun'
  f = "Double quoted \" string and some ' for fun"
  g = '''Multi-simple quoted ' string and some " for fun'''
  h = """Multi-double quoted " string and some ' for fun"""
```

And the code I used to show the highlights:

```
from redbaron import RedBaron
red = RedBaron(open('foo.py').read())

red[0].findAll('string').help()
red[1].findAll('string').help()
```