Fix options bug; add ability to skip missing migrate sources

[briandominick]

The options: setting for migrate actions was reading an erroneous truthiness to test for the existence of a setting. Fixed this so opt settings other than inclusive: false actually work.

Added the ability to set an option missing: with either warn or raise as values.

• missing: warn: the operation skips the migrate step and logs a warning to console.
• missing: skip: the operation skips, only warning under --verbose.
• missing: exit (default, backward compatible): the operation throws an error and exits.

Added example and explanation for new missing: setting to README and improved the illustrative capacity of the examples.

Thanks for help from Hector Palacios (@hpalacio).

[briandominick]

Adjusted the options for missing: as per @hpalacio's suggestion in PR #61

[hpalacio]

Cool! Will you be able to merge it today? Thanks.

[briandominick]

@hpalacio Merged and gem 0.9.5 published!