feat(command): derive missing command string from module filename

[nexdrew]

Mentioned in #494, this PR makes a command string optional for module-based commands, using the base filename of the require()d module for the command when the command string is not specified/exported.

Not only does this get us one step closer to implementing #492, but it also paves the way for supporting an "auto" mode for commandDir(), where conventions can be used to determine subcommand hierarchies and command names can be defined by filenames of submodules.

In this implementation, a command string is derived by filename via the following:

• For .command(): filename is looked up in require.cache via the new which-module dependency
• For .commandDir(): filename is already available via the require-directory API

This logic only applies when the command module does not export a command string. For non-module command objects (ones that are not require()d), a descriptive error is thrown if a command string is not given.

[coveralls]

Coverage remained the same at 100.0% when pulling 363e181 on derive-command-from-filename into 365fb9a on master.

[bcoe]

Looks good to me