`FindInMap` should not allow `If` as a parameter

Given something like:

``` python
FindInMap(
"MyMap",
Ref("SomeParameter"),
If('SomeConditional', 'key1', 'key2'),
),
```

When uploading this template to CloudFormation, it results in the following error:

> Template validation error: Template error: every Fn::FindInMap object requires three parameters, the map name, map key and the attribute for return value

In order to be processed successfully, it must be written as:

``` python
If(
'SomeConditional',
FindInMap(
"MyMap",
Ref("SomeParameter"),
'key1',
),
FindInMap(
"MyMap",
Ref("SomeParameter"),
'key2',
),
),
```

This is confirmed by the CF documentation at http://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/intrinsic-function-reference-findinmap.html under "Supported Functions", which states that the only functions that can be used inside of `FindInMap` are `Ref` or another `FindInMap`.

It would be nice if Troposphere could provide this "compile time" validation like it does for so many other things.