#315 FormattedStr

This pull request is for a feature request by Reiko.

FormattedStr is a new config type for entering string values.  It basically mimics the normal python string processing with the % notation.  So for example:
```
file_name : 
    type : FormattedStr
    format : realgal_type%02d_dilation%d.fits
    items :
        - { type : Sequence, repeat : 3 }
        - { type : Sequence, nitems : 3 }
```
The type of each item is determined from the format string, so the '%02d' item, for example, can only be one of the valid int_value types described on the Config Documentation page.  Only str, int and float are supported as valid item types.