Feature: Allow nested setting files

[javibravo]

Currently LambdaCron allows to set environment options in one file (~/.lambda-cron.yml). Could be good to be able to set environment options in different files (different projects).

Proposal:

1. Be able to include setting files in ~/.lambda-cron.yml. For example:

my-project-a:
    settings: /path/to/my-project-a/lambda-cron.yml
my-project-b:
    settings: /path/to/my-porject-b/lambda-cron.yml

[aarcro]

I'd prefer a command line flag to point to a config file, and/or looking at lambda-cron.yml in the current directory or ancestor.

But agree, the single hard coded config path is not flexible enough.