Support unmarshal into toml.Tree (pelletier#347)

Fixes pelletier#333

Support unmarshal into toml.Tree (pelletier#347)

Fixes pelletier#333

TOML 0.5.0 (pelletier#320)

go-toml now officially supports all TOML 0.5.0 features. If anything
does not work according to the spec, please file a bug!

Bump gopkg.in/yaml.v2 from 2.2.3 to 2.2.4 (pelletier#309)

Bumps [gopkg.in/yaml.v2](https://github.com/go-yaml/yaml) from 2.2.3 to 2.2.4.
- [Release notes](https://github.com/go-yaml/yaml/releases)
- [Commits](go-yaml/yaml@v2.2.3...v2.2.4)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Handle other key types in Unmarshal (pelletier#276)

Previously, this would fail with:

```
panic: reflect.Value.SetMapIndex: value of type string is not assignable to type toml.letter [recovered]
panic: reflect.Value.SetMapIndex: value of type string is not assignable to type toml.letter
```

Now this only panics when the key type cannot be converted from a
string.

Option to keep fields ordered when marshal struct (pelletier#266)

Adds a new `Order()` option to preserve order of struct fields when
marshaling.

Multiline Marshal tag (pelletier#221)

The new multiline tag works just like the existing 'commented' tag (i.e.
`multiline:"true"`), and tells go-toml to marshal the value as a
multi-line string. The tag currently has no impact on any non-string
fields.

Fix backward incompatibility for Set* methods (pelletier#213)

Patch pelletier#185 introduced a backward incompatibility by changing the arguments
of the `Set*` methods on `Tree`.

This change restores the arguments to what they previous were, and
introduces `SetWithComment` and `SetPathWithComment` to perform the same
action.

Fix example code in README (pelletier#187)

Write empty tables as well (pelletier#169)

Empty tables are allowed by the spec, so they should not be removed:

  [[empty-tables]]
  [[empty-tables]]

is perfectly valid.

Fixes pelletier#163