optional --destination param on mount

[ransomw1c]

following up on a request from @galvare2 , these changes allow omitting the --destination on bundle mount (and bundle mount new).

when omitted, a subpath of system's /tmp directory is constructed by golang's io/ioutil.

[kerneltime]

We can use the bundle hash and write the bundle descriptor last or write a done marker file. The next time we can skip downloading the files and reused the json files downloaded.. ?

[ransomw1c]

the bundle descriptor and filelists are written to disk at the same time it's used to initialize the in-memory structures. #218 contains the change to read ${destination}/.datamon/ folder from disk into in-memory structures.

since this implementation places the destination in a temporary directory that might vanish after the process exits, it's not clear where the reusable json files are to be stored... i suppose creating a subdirectory of ${HOME}/.datamon, say ${HOME}/.datamon/meta-cache, is a reasonable option?

creating a new issue to document the intent of these changes seems more likely to me than including them in this diff.

[kerneltime]

Needs conflict resolution..