Resolving strange charachters in non Mac terminal, fixes #399

Looks like the double inclusion of escaping characters (once in their definition, once in output line) was giving me trouble in Linux. Give it a try on a mac (or I will tonight).
@alrra & @mathiasbynens - it was when the definition escaped the var and we ended up with two `\[` in a row that I was seeing strange characters.

It's my understanding that 

```
hostStyle="\[${bold}${red}\]";
PS1+="\[${hostStyle}\]\h"; # host
```
was reading as 
```
PS1+="\[\[${bold}${red}\]\]\h"; # host
```
and ```\[\[``` was causing me issues.

FYI - This line worked fine:
```
PS1+="\[${bold}${red}\]\h"; # host
```