Add an option to sort includes

[oalders]

so that, for example, when tidying merge conflicts we can just toss out the re-used modules and end up with a consistent sort order.

Rules should probably be something like:

• Sort only contiguous includes. If they're broken out into different sections, just sort the sections
• Ignore pragmas
• Ignored modules can still be sorted
• Modules annotated with ## no perlimports should not be sorted
• All comment lines which precede an import statement should remain above that import after sorting