Added the Oniguruma regular expression library as an (optional, but enabled by default) dependency (#81). Oniguruma supports more regular expression syntax types than the POSIX regex API, and often performs better. To build bfs without this new dependency, do make WITH_ONIGURUMA= to disable it. Thanks @data-man!

Added support for the ed, emacs, grep, and sed regular expression types (#21)

Before executing a process with -exec[dir]/-ok[dir], bfs now ensures all output streams are flushed. Previously, I/O from subprocesses could be interleaved unpredictably with buffered I/O from bfs itself.