Configurable Key Parameters

Extract all key parameters as configurable, including:

    ele:min-grad: gradient threshold to produce binary map         
    ele:ffl-block: fill-flood threshold
    ele:min-ele-area: minimum area for selected elements 
    ele:merge-contained-ele: if True, merge elements contained in others
    text:max-word-inline-gap: words with smaller distance than the gap are counted as a line
    text:max-line-gap: lines with smaller distance than the gap are counted as a paragraph