New commands, small improvements

• Don't explicitly define a docker-compose.yml file, to allow default docker-compose.override.yml to be used (compose version 3 feature) via #81
• Make the dump command a little cleaner by using MySQL environment variables to avoid Warnings when running a dump via #68
• Added a php command shortcut: ./vessel php -r "echo 'hello world';"
• Added checks to see if env vars existed in the .env file before attempted a find/replace against them via #61
• Increased PHP max upload size via #59
• Added new commands via #60
  • ./vessel php ... - execute arbitrary php commands in the container
  • ./vessel node ... - execute node inside the container (previously only npm, yarn had a command)
  • ./vessel ssh [app|mysql] - executes "bash" inside the container (note: it's not actually SSH)
  • ./vessel mysql - log into the mysql database and set the current database to the one set by env vars (so you won't need to run use some_db in the mysql shell)