[feature] auto create directory when export a model

[tskdsb]

Is this a BUG REPORT or FEATURE REQUEST?:

Uncomment only one, leave it on its own line:

/kind bug

/kind feature

What happened:
when I try to export a model after pulling, I got an error:

ormb export -d bbb 1.2.3.4:5678/test-modelset/test-modeset:v1
ref:       1.2.3.4:5678/test-modelset/test-modeset:v1
digest:    c860afd8f993a754b9910f40fb66e45005988c0e23c7470560a2dd9033843f07
size:      28.9 MiB
format:    MXNETParams
Error: open /aaa/bbb/ormbfile.yaml: no such file or directory
...

I can't get the reason by the message, but when I created a directory called bbb, it exports successfully. So, it needs the export-directory to exist firstly.

What you expected to happen:
create the directory when I run ormb export ... like git clone

How to reproduce it (as minimally and precisely as possible):

Anything else we need to know?:
I may provide a PR to solve it