RFC: Human readable duration parsing for Timex.Duration

Similar to Ruby's ChronicDuration gem, I need an elixir library that will support parsing natural language. Since Timex can output in a human readable format, it seems like it would make sense to be able to parse in such a way as well. Thoughts?

```ruby
require 'chronic_duration'
=> true
>> ChronicDuration.parse('4 minutes and 30 seconds')
=> 270
>> ChronicDuration.parse('0 seconds')
=> nil
>> ChronicDuration.parse('0 seconds', :keep_zero => true)
=> 0
```