Feature Request: Add -e/--env flag to specify environment files

Problem

Currently, Overmind only supports loading environment files from specific hardcoded locations:

• ~/.overmind.env
• ./.overmind.env
• ./.env (unless OVERMIND_SKIP_ENV is set)
• Files specified via OVERMIND_ENV environment variable

While the OVERMIND_ENV environment variable allows specifying custom env files, there's no CLI flag equivalent. This makes it inconsistent with similar tools like Honcho, which provides a -e/--env flag for this purpose.

Proposed Solution

Add a -e/--env flag to the start command that allows specifying environment file(s) to load, similar to Honcho's implementation:

# Single file
overmind start -e .env.local

# Multiple files (comma-separated)
overmind start -e .env.local,.env.development

# Long form
overmind start --env .env.local,.env.development

The flag should:

• Accept comma-separated file paths (like Honcho)
• Load files after the default env files (allowing override)
• Maintain backward compatibility with OVERMIND_ENV environment variable
• Support the same environment variable name (OVERMIND_ENV) for consistency

Use Cases

• Different environment files for different environments (development, staging, production)
• Project-specific env files without modifying global or default .env files
• Easier integration with CI/CD pipelines where env files might be in non-standard locations
• Consistency with other Procfile-based tools like Honcho

Example

# Current way (works but requires environment variable)
OVERMIND_ENV=.env.local,.env.development overmind start

# Proposed way (more intuitive and consistent)
overmind start -e .env.local,.env.development

References

• Honcho's -e/--env flag: https://github.com/nickstenning/honcho