Filter downloads by mime

Allows filters in the config file to stop storing files of specific mime types

Use Case:
Maltrieve gets lots of HTML pages that i don't want in my viper instance.

add to maltrieve.cfg

```
mime_block = text/html, text/plain 
```

Anything matching the mime by magic bytes is logged but not stored or processed any further.

```
2014-09-28 11:23:01 140329684920064 text/html in ignore list for http://shwmf.net/efckt/
```