add creation-date-key modification-date-key` configuration options

[y86-dev]

These two configuration options are used to change the way zk obtains the creation date and modification date from a file.

I have a lot of notes that use created and changed as the keys for the creation and modification date (I store them in git and thus the true creation/modification date are useless).

This also is useful when one wants to use a different language for the keys in the frontmatter.

I am pretty new to go and this is my first contribution to zk, so I very likely have missed something.

[y86-dev]

Ah I just noticed that I didn't finish the tesh files correctly... I'll get to that tomorrow, never used tesh before and the make tesh command didn't result in any errors, so I didn't know what the correct output should be.