Implement dynamic string size.

[mattcollier]

Allows for strings of variable lengths to be passed in.

Since I didn't see any existing in-line comments, I did not add any.

I'm not sure what the safety implications are for using Box in this manner. Here's a recent discussion that seems relevant: rust-lang/unsafe-code-guidelines#157

Passing NULL into napi_get_value_string_utf8 will return the size of the string.

https://nodejs.org/api/n-api.html#n_api_napi_get_value_string_utf8

Adapted from example provided here: https://github.com/nodejs/node-addon-examples/pull/107/files#diff-8252f94e35e4da5136727f8d25a119308698c527d26c04f4dc71ce90ca8fc85b

That PR is in connection with this issue: nodejs/node-addon-examples#83 (comment)

[sehz]

This looks good.

Can you add test cases for in examples/function? as well as bump up version for crate.

Thanks!

[sehz]

Please provide test case in examples/function

[sehz]

Nice job on tests