Duration scale for notes

Currently, the duration scale for a note is seconds.

That is that you can tell banjo to play a specific notes for X seconds.

```
# Example:
play(c(4)).for(5).at(4)
```

To be consistent with sheet music, I think this should change to whole, half, quarter, eighth, etc.

```
play(c(4)).for(:half).at(4)
```

Or:

```
play(c(4)).half.at(4)
```
