Custom meta-data block format. Fixes #1753

This pull request adds the possibility to set your own meta-data block format.

As extra option it has the ability to remove the lines that are not filled by the new script window.

Some advantages of this pull request: 
* add your own @-attributes (like the `@require` or `@author` attribute).
* some people want tabs instead of spaces.
* you can also rearrange the order of the @-attributes.
* already write part of the script below the meta-data block.

Predefined variables:
* `%name%`
* `%namespace%`
* `%description%`
* `%include%`
* `%exclude%`