add controls for font cache to ``luaotfload-tool.lua``
rename option ``--flush-cache`` -> ``--flush-lookups`` to avoid confusion with cache control
update manpage
rename ``flush_cache`` -> ``flush_lookup_cache``
implement ``--dry-run`` for filesystem tracing
add ``--dry-run`` to usage info
[doc] add thanks section
Merge pull request #63 from phi-gamma/master

extend luaotfload-tool; thanks in doc