v0.9.0

The 0.9 series should be the final version of tome before a 1.0
release.

This includes a couple breaking changes, and users should update
their scripts:

• the following commands are now builtins, and will overrides any scripts with the same name: commands, exec, help, tome.
• tome no longer infers the shell from the SHELL environment variable in
  any situation: users should pass in the desired shell directly during the initialization.
• a script must be explicitly be marked with COMPLETE to support
  completion. Previously all scripts were assumed to support completion.

What's Changed

• Adding reserved commands by @toumorokoshi in #31
• adding support for exec by @toumorokoshi in #32
• Require "COMPLETION" flag to complete script by @toumorokoshi in #34
• require shell to be passed in by @toumorokoshi in #35
• changing flag for completion support to COMPLETE by @toumorokoshi in #36

Full Changelog: v0.8.1...v0.9.0