Add advanced usage options/functionality

[tbonfort]

• The gdal ghost area optimisations can be disabled, and will not be emitted for PlanarConfiguration=Separate, as it is not supported by gdal. Fixes #8
• Each IFD may now reference only a single mask
• The IFD structure is now exported and can be used to hand-craft an expected COG from dynamic data
• Encoding (little vs. big endian) is now configurable
• For PlanarConfiguration=Separate files, it is possible to fine tune the layout/interlacing of each plane's tiles and mask tiles, supporting tile orders like r1,g1,b1,r2,g2,b2 (the default), or r1,r2...g1,g2...b1,b2 or even r1,g1,r2,g2,...rn,gn,b1,b2

[coveralls]

Pull Request Test Coverage Report for Build 2102149321

• 384 of 549 (69.95%) changed or added relevant lines in 3 files are covered.
• 25 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+2.4%) to 53.865%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
loader.go	30	42	71.43%
field.go	23	89	25.84%
cog.go	331	418	79.19%
<!--	Total:	384	549	69.95%	-->

Files with Coverage Reduction	New Missed Lines	%
loader.go	1	70.67%
field.go	4	20.33%
cog.go	20	71.9%
<!--	Total:	25		-->

Totals
Change from base Build 1520222711:	2.4%
Covered Lines:	662
Relevant Lines:	1229

---

💛 - Coveralls