lfs v2.1.0

• fix failure in parsing --cols arguments with underscore
• list all column names in case of bad value of --cols
• improve alignement of the 'inodes' column - Fix #38
• it's now possible to have size info but no inodes, so as to be compatible with filesystems not filling inodes info in a consistent way - Fix #36
• breaking change: inodes related fields in the JSON have been moved to a dedicated inodes struct (because they're now optional)
• addition of 2 new non default columns: use_percent and inodes_percent
• switch columns type and disk: it makes more sense to have the type of filesystem just after the filesystem name