Adding an overlapping dates function to Timex.Interval

`Timex.Interval` Has a handy `overlaps?` function that helps us know if two intervals overlap each other. I wanted to propose a new function `overlapping_dates` which creates a new interval with the actual dates that overlap.

Something along the lines of

```elixir
def overlapping_dates(a, b) do
if Timex.Interval.overlaps?(a, b) do
Timex.Interval.new(
from: latest_date(a.from, b.from),
until: earliest_date(a.until, b.until)
)
else
[]
end
end

defp earliest_date(a, b) do
if Timex.before?(a, b) do
a
else
b
end
end

defp latest_date(a, b) do
if Timex.before?(a, b) do
b
else
a
end
end
```

Though it is trivial to create this function with the helpers Timex already provides I thought it would be handy if it was already part of the library.