Add flag to toggle Bashmarks interoperability

[techwizrd]

There isn't necessarily a need to restrict Fishmarks to only bashmarks functionality, bookmark names, error messages, and help text. There should be a flag like BASHMARK_COMPATIBLE to make Fishmarks act exactly like Bashmarks. Unseting the flag should allow Fishmarks to provide less restrictions on bookmark names and more helpful error messages and help text.

For example, UNIX directories can have hyphens in their name, but currently hyphens are not allowed in bookmark names. Hyphens are not allowed because they cannot be used in bash environment variables and bashmarks saves all of it's directories as environment variables prefixed with DIR_ in the ~/.sdirs file. This is only one of the many restrictions that could be lifted if Fishmarks did not have to keep strict compatibility with Bashmarks.