Add a 'quote' attribute to generate quoted TS properties

[samkearney]

In TypeScript it is possible to have interfaces with fields that are not valid JS identifiers, if they are enclosed in quotes. If you use the current version of tsify with #[serde(rename)], it can produce interfaces with these properties, but in most cases it does not enclose them with quotes (unless they contain a -, with current logic).

One approach to fixing this is to enhance the logic for automatically detecting whether a property is a valid JS identifier. Since this is defined with a relatively large category of Unicode characters, the easiest way to do this would be to take a dependency on a crate such as unicode-ident.

I figured a simpler way was to give the crate user an escape hatch in the form of a new attribute quote which will enclose the resulting TS field in quotes.

Closes #37