add utility class OptionsPrefix
OptionsPrefix: mark method prefix() with const
OptionsPrefix: add method prefixed(option)
OptionsPrefix constructor: default prefix =""
fix help massage for --workers
add option --debug not to catch exceptions to preserve stack
move file selection features from Output to AbstractOutput

AbstractOutput inherits Process and OptionsPrefix
add class AbstractOutput to global.hpp
Output.cpp: include <ostream>
fix AlignmentRow::nearest_in_fragment and add test case
copy boost/assert.hpp from up-to-date boost for BOOST_ASSERT_MSG, customize it
AlignmentRow.grow: use BOOST_ASSERT_MSG with readable message
add class PrintOverlaps
add tool overlaps (pipe for class PrintOverlaps)
apply astyle to boost-assert.hpp
fix long line in AlignmentRow.cpp
improve style of PrintOverlaps
PrintOverlaps.print_block(): move part of code to local function
PrintOverlaps: insert one space to move position 0 to next column after '|'
PrintOverlaps: fix width (-=2)