Add --silent option for text output in magic

Sometimes MATLAB functions print quite a bit of text to stdout. This
flag prevents this from being rendered in the Notebook.

In this example, the output from forgetting semicolons will not be
displayed:
```
%%matlab --silent -o x
a = [1, 2, 3]
x = a * 3
```
Merge pull request #55 from aebrahim/silent_option

Add --silent option for text output in magic