pass "Options" object on to fpCalc

This allows specifying the "command" property in the options
document options being passed to fpcalc
documented "command" option
Merge pull request #2 from Earl-Brown/patch-1

pass "Options" object on to fpCalc
Merge pull request #3 from Earl-Brown/patch-2

document options being passed to fpcalc
Merge pull request #4 from Earl-Brown/patch-3

documented "command" option
Options object is not optional
Ensure existence of options.key
Add `options.fpcalc`, used as fpcalc options

Add optional `options.fpcalc` which is passed directly to the `fpcalc()`
call. This can be used to set any fpcalc options. See
https://github.com/parshap/node-acoustid/pull/2.

Also improve options documentation in readme.
1.2.0
Lock down dependency versions
1.2.1