fiwalk: Allow plugins to process some virtual files

A boot-sector virus was not being detected by a Fiwalk plugin.  The
virus was detected when directly scanning the disk image, and when
scanning the boot sector extracted from the image.

This patch allows Fiwalk to pass a specific virtual file to plugin
processes.

Reported-by: Christie Peterson <cpeterson@jhu.edu>
Signed-off-by: Alex Nelson <a.nelson@prometheuscomputing.com>
Change mmls to 3 digit precision so that it aligns nicely when there
are >= 100 partitions.
Minor space correction.
README update to Markdown
Merge pull request #381 from sroberts/patch-2

README update to Markdown
manual application of #380 - file was renamed to md
Merge pull request #378 from dkarpo/develop

Change mmls to 3 digit precision so that it aligns nicely when there are >= 100 slots.
Merge pull request #358 from ajnelson/fiwalk_plugins_on_virtuals

fiwalk: Allow plugins to process some virtual files
Fix for #383
Merge branch 'develop' of github.com:sleuthkit/sleuthkit into develop
Applied patch from Hin-Tak Leung to not consider this ZLIB code to be a fatal error.