Evaluate Interface for Custom Tags

[phip1611]

I've seen people using custom Multiboot2 tags to transfer data from a custom bootloader to their kernel. We should evaluate if and how we want or can support this.

[blitz]

To formulate this as a usecase: As a user of this library I would like to be able to support custom tags. The library should not limit the user to the tags in the current spec and allow the user to define completely custom tags.

[IsaacWoods]

I think this could be achieved fairly easily by exposing BootInformation::get_tag (which imo should be renamed tag but that's a nit) and reworking TagType to support custom values (e.g. remove the repr(u32), add a Custom(u32) variant, and adding manual From<u32> and Into<u32> impls)?

[phip1611]

Closed with #118