Seamusabshere dotenv dash f option

This refactors the `-f` support added in #127 to avoid shelling out.

/cc @seamusabshere 

Before:

```
$ rake
Finished in 3.82 seconds (files took 0.21575 seconds to load)
```

After:

```
$ rake
Finished in 0.11085 seconds (files took 0.21908 seconds to load)
```