Improve startup time

Starting fast is critical to the usability of command line apps. Unfortunately, Escort takes about 450-500ms to boot on my machine.

I've replaced all the `require`s in escort.rb with `autoload`, so apps only pay the startup time cost for things they use.

With autoload:

```
λ time ruby -I./lib examples/basic
....

real	0m0.076s
user	0m0.062s
sys	0m0.013s
```

```
λ time ruby -I./lib examples/suite_complex --help
....

real	0m0.097s
user	0m0.077s
sys	0m0.018s
```

Without autoload:

```
λ time ruby -I./lib examples/basic
....

real	0m0.468s
user	0m0.427s
sys	0m0.038s
```

```
λ time ruby -I./lib examples/suite_complex --help
....

real	0m0.469s
user	0m0.426s
sys	0m0.040s
```

Also I dunno what's the deal with the commented out stuff in `spec/support/shared_contexts/integration_setup.rb`. The specs were failing for me so I commented out the `$stdin` assignment and everything passed lol