Add `Action.getenv` and use variadic options in `New()`. (#19)
* Adding `Action` options.
* Using options in `New()`.
* Adding `Action.getenv` and `OptGetEnv` helper.
* Adding godoc for `Opt*()` helpers.
* Adding `getenv` default in `New()`.
* Replacing `os.Getenv` usage with `Action.getenv`.
* Ditching `Action.setEnv` in favor of using `OptGetenv`.
* Ditching `Action.addPath` in favor of using `OptGetenv`.
* Ditching `Action.getInput` in favor of using `OptGetenv`.
* Ditching `Action.issueFileCommand` in favor of using `OptGetenv`.
* Ditching `NewWithWriter` in tests.
This will make it easier to deprecate `NewWithWriter` later.
* Adding test for `NewWithWriter`.
* Fix year in new `options_test.go`.
* Modifying `Option` so it also returns an `Action`.
* Adding a `nil`-guard for `opt` in `New()`.
* Adding deprecation notice to godoc for `NewWithWriter()`.
* Renaming `Opt*` to `With*` for options.
The diff was completely generated via:
```
git grep -l OptWriter | xargs sed -i '' s/OptWriter/WithWriter/g
git grep -l OptFields | xargs sed -i '' s/OptFields/WithFields/g
git grep -l OptGetenv | xargs sed -i '' s/OptGetenv/WithGetenv/g
```
so reviewing the whole diff is not necessary (if that makes code
review easier).
* Exporting `getenvFunc`.
Done via:
```
git grep -l getenvFunc | xargs sed -i '' s/getenvFunc/GetenvFunc/g
```
