Add -e/--env flag to specify environment files

[kler]

Implements a CLI flag to specify environment files, matching Honcho's behavior. This allows users to specify custom env files via command-line flag instead of only through the OVERMIND_ENV environment variable.

Changes:

• Add EnvFiles field to start.Handler struct
• Add -e/--env flag to start command (maps to OVERMIND_ENV env var)
• Add loadEnvFiles() method to parse and load comma-separated env files
• Load env files from flag before processing other options

The flag accepts comma-separated file paths and loads them after the default env files (~/.overmind.env, ./.overmind.env, ./.env), allowing flag-specified files to override default values.

Usage: overmind start -e .env.local overmind start -e .env.local,.env.development overmind start --env .env.local,.env.development

Closes #206