Better dev watch customization support

[stephendolan]

In adding translations, I realized that the dev server wasn't restarting when I changed the content in config/locales/**/*.yml.

In searching, I found that we hard-code the set of files that can trigger a recompile here:
https://github.com/luckyframework/lucky/blob/master/tasks/watch.cr#L212

I propose adding a new setting here that contains all of the default values in a YAML array, and allows for easier per-application customization:
https://github.com/luckyframework/lucky/blob/master/src/lucky/server_settings.cr

We'd also need to modify the generated apps to contain the default files to watch:
https://github.com/luckyframework/lucky_cli/blob/master/src/web_app_skeleton/config/watch.yml

I'm thinking the content would look something like this:

host: 127.0.0.1
port: 5000
watch_patterns:
  - "./src/**/*.cr"
  - "./src/**/*.ecr"
  - "./config/**/*.cr"
  - "./shard.lock"

[jwoertink]

Related: #920

[paulcsmith]

Yeah I think ideally we'd make it so you can configure the watch task with Habitat. The problem is the watch task is precompiled so wouldn't take into account the configuration changes unless you deleted the preocmpiled binary and then recompile it.

We could add a comment to the crystal file to tell them to delete X file and tell them how to recompile the watch task. That might work