Support nested placeholders

[Dan Kelly]

So we can do something like:

{{data.link1:<a href="{{data.link1}}">Test link</a>}}

And the link is only displayed if the variable is defined.

[Dan Kelly]

Requires #23

[Dan Kelly]

As per #23 (comment)

For example we should be able to replace:

data.key  = "has value"
data.key2 = "also has value"
data.key3 = "maybe"

  {{data.key text to use if the {{data.key2 inner key {{data.key3}}}} has content}}
= {{data.key text to use if the {{data.key2 inner key maybe}} has content}}
= {{data.key text to use if the inner key maybe has content}}
= text to use if the inner key maybe has content

[Dan Kelly]

Closed via #61