Allow custom properties, which go below the symbol body

[dnschneid]

Custom properties can have an asterisk in the name to make them hidden.

Hopefully satisfies #77, although it's not exactly the same.

[devbisme]

Thanks for all the fixes/additions! The master branch now has them all and it passes my unit tests and examples, but those don't really test your additions for functionality. So do some of your own tests and let me know if it's good to go.

[dnschneid]

Thanks for accepting everything! Looks like just one thing was missing (probably my fault): #97.

[devbisme]

No problem!

I did just change the way the overwrite and merge operations are handled by moving the logic into the row_file_to_symbol_lib function. So you may want to check that to make sure it operates as you think it should.

[devbisme]

Note that the unit tests don't check for most of the changes made by your PRs, and I had to resolve merge conflicts when I was incorporating them. So check carefully that everything is working as you expect.

[dnschneid]

Yeah, I initially diffed against my combined branch and things lined up (except for #97), so your resolutions were good. I also tested against the same files I was testing against.