Add support for namecheap dns-01 challenge

[ghost]

Use case

I noticed dns-01 challenge for the ACME client was recently added in 93cf4f2, this issue (selfishly) suggests adding the namecheap dns provider.

Your idea for a solution

There is already a module for libdns namecheap, so integration with maddy should be fairly straightforward. If this suggestion has support, I'd be happy to open a PR to add it.

• I'm willing to help with the implementation

[foxcpp]

Just pushed c0845fb to master that adds namecheap provider.

[foxcpp]

Note to self: Using github.com/libdns/namecheap increased minimum Go version to 1.16 (before it was Go 1.14.3). Documentation, etc need updating.