Ignore field options (omitempty,omitzero) during Decode

[chowey]

This fixes #89. Currently the only options are omitempty and omitzero, which have no significance while decoding, so these can simply be ignored.

I thought it best to use the same getOptions function so this stays unified in the future (if some additional options are added later).

[buaazp]

Yes, I meet this bug and @chowey 's PR fixed it.

If some fields has ,omitempty option, they will be ignored when decoding.

For example, in the _example/_example.go, if I modified codes to:

type ownerInfo struct {
    Name string
    Org  string `toml:"organization,omitempty"`
    Bio  string
    DOB  time.Time
}

And then run it:

./_examples
Title: TOML Example
Owner: Tom Preston-Werner (, GitHub Cofounder & CEO

You can see the Org is blank now.

@BurntSushi plz merge these codes into master branch.

[chowey]

Just noticed this is related to #82. I did the check in a different place.

[titanous]

This could use a test to ensure that there isn't a regression.

[chowey]

I added a test.

[ayufan]

@BurntSushi Could you merge this?

[cespare]

I went with #82 instead.