Add default read offsets for Node Buffer compatibility

[benallfree]

This PR adds an optional offset to the readXX methods to create compatibility with the Node Buffer API https://nodejs.org/api/buffer.html#buffer_buf_readbigint64be_offset

[mcollina]

could you please add a unit test? Thanks!

[benallfree]

@mcollina Hey ok I added some tests. Please check out how I handled the floats, I kind of feel like this is a better test anyway. Rather than testing against hard-coded hex values, we can test equivalence with the output value of a real Node Buffer for the same input. If you agree that this is a better test, I'm happy to update the rest of the read* tests to match. Please let me know! :)

[mcollina]

lgtm

[rvagg]

sure, I don't think this is a breaking change in any way, so just a semver-minor?

[benallfree]

@rvagg agree, just minor or even patch

Did you want me to update the readXX tests to test against a native Buffer output rather than hard-coded values?