Enhancement: support custom input ICC profiles

[lovell]

Embedded input profiles (and default profiles for CMYK images that are missing a profile) are already supported. This enhancement relates to using custom input profiles.

Proposed API:

sharp(input, { profile: '/path/to/profile.icc' })
  ...

The icc_import operation can be used for this (use XYZ profile connection space and set embedded to FALSE).

See #1323 for custom output profile support.

See #218 for previous discussion on this and related features.