Implement a .subrc file sourcing where EXX_INSTALL_PATH and EXX_SYMLINK_PATH can be specified as variables

[CaptainQuirk]

• Exx should test for the existence of a ~/.subrc file
• The file can contain global variables declaration.
• Variables to use in the install subcommand should be
  • EXX_INSTALL_PATH: for the path where the exx source should be moved
  • EXX_SYMLINK_PATH: for the path where the main executable should be symlinked
• After sourcing the file, if those variables were set, use them directly
• If not, prompt the user for those values and go on with the install process
• At the end of the installation, advise the user to create the ~/.subrc file with the values he/she entered