fix(core): Allow to use `time.Duration` struct fields

[liranp]

Description

We are trying to use this package to initialize a time.Duration field but getting an error when the answer is written to the target struct by core/write.go#L189-L190. This PR adds support for struct fields of type time.Duration.

Example

type Options struct {
    Timeout time.Duration
}

survey.Ask([]*survey.Question{{...}}, new(Options))

• Answering with a string in an integer format (for example, 30) returns the following error: reflect.Set: value of type int64 is not assignable to type time.Duration

• Answering with a string in a valid duration format (for example, 30s) returns the following error: strconv.ParseInt: parsing "30s": invalid syntax

[AlecAivazis]

Nice! Looks like we have an issue in travis that's preventing your build from passing. I'll try to figure out what's going on so we can get this merged ASAP

[AlecAivazis]

Okay so the latest PR should fix your build problem. Mind rebasing on the latest master?

[liranp]

Done. Thank you!