v2: chainable options API

[moorereason]

v2 (40cfb6f) doesn't support chaining of configuration options when building an encoder/decoder. Was that intentional?

For example, these two blocks accomplish the same thing:

// v1
err := toml.NewEncoder(buf).ArraysWithOneElementPerLine(multi).Encode(v)

// v2
enc := toml.NewEncoder(buf)
enc.SetArraysMultiline(multi)
err := enc.Encode(v)

[pelletier]

This was not intentional! Adding this to the list of things to do after I figure out this Document feature.

[pelletier]

This has been fixed in #670.