Truncate the end of long token titles

By changing the autoresize rules on the token label, we can force the
label to appropriately size itself to the token title. Before, a
token’s label was sized to its content, meaning that if it was actually
longer than a line in the token field, it freely overflowed and was
clipped by the outer VENToken’s view. I also exposed the line break
mode for VENTokens as a whole, allowing a client application to change
the line break mode to one appropriate to their data (breaking in the
middle or beginning of the line, for example). Including this in a release
requires a minor version as it adds new API.
I prefer instance properties to avoid unintentional side-effects. At least in the Venmo app, we use `VENTokenField` in two separate places, and it could be the case that we want to use different line break styles for each (or at least have full control of its behavior in each context).

Let me know what you think, and thanks for the pull request! :smile_cat: