Add support for object __rest helper when using custom typescript 2.1

[tabanliviu]

Typescript 2.1 introduced support for object spread and rest

Using a custom typescript library fails because the __rest helper is not added to the build file (see here)

PS: this does bring up an important issue for future compatibility with other features that require helpers. Is there a better way planned to load helpers from typescript directly instead of copying them in the plugin source?

[MattiasBuelens]

TypeScript now has support for an external helpers library published as tslib on npm (also on GitHub) by using the --importHelpers compiler option. It may be interesting if this rollup plugin could use this new compiler option when using TypeScript 2.1 or higher, or at least pull the helpers from the official tslib package (perhaps as a peer dependency?).

[PaulBGD]

Closing issues that I've integrated into #91, will be merged into master soon.