Fix regex escape punctuation signs

Before:
![screen shot 2013-05-28 at 09 55 08](https://f.cloud.github.com/assets/2074433/572285/f793f24a-c795-11e2-91ea-67de1e86addc.png)
After:
![screen shot 2013-05-28 at 09 54 28](https://f.cloud.github.com/assets/2074433/572286/fb274ef2-c795-11e2-9054-0e96e5e5a36f.png)

Note a quantifier has the same highlight of their escaped version. However, this is caused by the highlighting that sets the same color for them.

As a side effect, this solves another issue with punctuation signs inside syntax regions such as `elixirInterpolation`.

Before:
![screen shot 2013-05-28 at 09 58 32](https://f.cloud.github.com/assets/2074433/572302/50e1dc18-c796-11e2-9941-5287c5a86b63.png)
After:
![screen shot 2013-05-28 at 09 59 16](https://f.cloud.github.com/assets/2074433/572307/697b1a3c-c796-11e2-8498-59266893d793.png)

Nice way to check syntax `echo map(synstack(line('.'), col('.')), 'synIDattr(v:val, "name")')` with the cursor on the thing you want to check.