Treat `.mli.foo` files as `.mli` files

[lpw25]

We sometimes have files with extensions like .mli.pp. It seems that merlin does not recognise the extension in such cases and defaults to treating them as .ml files. It would be mildly useful for it to treat them as .mli files instead.

[trefis]

We already have a mechanism to support custom extensions, but you have to specify them in your .merlin file, for instance:

SUFFIX .ml.foo .mli.foo

(one pair of extensions per SUFFIX directive).

[trefis]

Note: this is "additive", you can add it to your .merlin unconditionally, regular .ml and .mli files will still be handled as normal.

[lpw25]

I guess that we could use that. It seems a bit of a shame to add multiple SUFFIX directives to every .merlin file we make.