Add gitlog command (fixes #56)

You're right. I didn't think that through enough. There is no way to map a customValueFn to two args. 

You may combine the to and from args such that they expect a string:
```js
options.toFrom = 'v0.1.0...HEAD';
```
Another approach would be to apply this logic on the individual arguments and combine them **before** instantiating the argUtil instance:
```
...
if (options.from) {
    options.from = options.from + '..' + options.to || 'HEAD';
} 
argUtil = new ArgUtil(this, argConfigs);
...
```
A third option would be to extend flopmang to accommodate this case, but that is probably well out of scope. 

Let me know what you think.