Add some options to optimize pretty-printing

Created by: Ede123

This PR adds three new options:

• --nindent which allows to set the indentation depth (e.g. 2 spaces) therefore giving the user the possibility to adjust the pretty-printing to his personal preferences.
• --no-line-breaks which suppresses line breaks (and obviously also indentation) in the output altogether. This closes #10 (closed) and adds a possibility for a "minimal" output.
• --strip-xml-space tells Scour to remove the xml:space attribute from the root SVG element. At least one popular vector graphics editor adds xml:space="preserve" to the root SVG element thereby thwarting our efforts to pretty-print the output completely.