More permissive parsing of unknown load commands

[woodruffw]

ruby-macho currently throws an exception (LoadCommandError) when it can't resolve a load command type to a class.

To prevent the future addition of load commands by Apple from fouling up the parser, we could allow the user to toggle more permissive behavior:

• If the load command ID corresponds to a known class, instantiate that class
• If the load command doesn't correspond to a known class, instantiate the generic LoadCommand class

We already hard-code this behavior for LC_PREPAGE, since Apple doesn't provide a public structure for it. This behavior could be made generic fairly easily.

[woodruffw]

A potential problem: it's not really clear where to draw the line for permissive parsing -- there are lots of other little fields in a Mach-O binary that could be wrong but non-fatal (like cputype and cpusubtype). Thus, the permissive parser option needs to be opinionated: do unknown CPU types and header flag combinations cause an exception, while unknown load commands get a pass?

[woodruffw]

Decided to go with just permissive load command parsing for now (non-default, the default remains strict) -- making other fields permissive is likely to lead to lots of problems.