Adding TOML support

[hairyhenderson]

This adds:

• support for (unofficial) MIME application/toml for --datasource
• TOML parsing for datasource function
• new functions toml and toTOML

There's a catch though... It seems the library I'm using can't handle map[interface{}]interface{} like the YAML & JSON libraries do. So this fails:

$ gomplate -i '{{ `{"foo":{"bar":true}}` | json | toTOML }}'
2017/06/04 21:43:39 Unable to marshal map[foo:map[bar:%!s(bool=true)]]: toml: cannot encode a map with non-string key type

To work around this, I've patched the lib (see BurntSushi/toml#179) and am vendoring my branch until that's merged.

Signed-off-by: Dave Henderson dhenderson@gmail.com