Allow procs to be passed to the watch method

It would be useful if a `proc` could be passed as the second argument of the `watch` method to allow custom commands based on the name of the changed file. For example:

```crystal
custom_command = ->(file : String) : String? do
# ... build a command based on the changed file, return the string
end

Watch.watch "./**/*.cr", custom_command
```

An example usage could be running the respective spec file for any file in the src/ dir:

```crystal
# runs matching spec for any file in /src
run_spec = ->(file : String) : String? do
spec = file
.sub(/^\.\/src\//, "./spec/")
.sub(/(_spec|_flow)?\.cr$/, "_spec.cr")

if File.exists?(spec)
puts %(> running "#{spec}")
"crystal spec #{spec}"
else
puts %(> matching spec "#{spec}" does not exist)
end
end

Watch.watch "./**/*.cr", run_spec
```