input checking for db.set

felix,

i'm not sure what your end goal with dirty is, but i definitely respect your vision to keep it small. that said, there are some cases for which you might want to check inputs.

specifically:

```
db = require( "dirty" )( "test.db" );
db.set();
db = require( "dirty" )( "test.db" );
```

results in `Error: Could not load corrupted row: {}`.

also,

```
dirty = require( "dirty" );
db = dirty( "test.db" );
db.set( "someKey", function(){} );
db.get( "someKey" ); // [Function]
db = dirty( "test.db" );
db.get( "someKey" ); // undefined
```

the function will be persisted to memory but not to disk, meaning that the two are no longer consistent. this is clearly an edge case, but i was wondering where you see taking dirty in terms of preventing this kind of disparity/corruption.