add ability to specify options to backup

[kdebisschop]

Resolves issue #80

• Adds loop to templates/restic_script_Linux.j2
• Adds usage info to README.md
• Fixes a few spelling and grammar errors in README.md

[kdebisschop]

It turns out that this is sort of unnecessary. You can actually just specify additional options in the list of sources ... they will just get passed along through that loop.

But there is also a quoting problem with sources.

[kdebisschop]

You can't quite stuff the variables in sources because sources are tied in with forgetting.

I have a more workable patch coming.

[kdebisschop]

I ran into another issue that I had to fix testing this. We have multiple database servers, each responsible for their own backups. Restic handles this fine, but forgetting requires an exclusive lock, so if the forgetting happens when one of the other servers is still backing up, it throws an error. I added a conditional to skip the forgetting for the case when multiple servers operate in parallel. I will need to generate another script to do the forgetting at a time different than the backups.

I made the default behavior unchanged, so it does not really affect people using a single source, but does allow the option of running with several hosts in parallel without throwing errors.

[kdebisschop]

does not handle files-from quite correctly (the script is not synced because there is no source)

[johanvdw]

Your PR is marked as a draft: is there a reason for that?
I'm also interested in this option (to specify "--one-file-system").

[kdebisschop]

I did some cleanup...so I marked it draft until I was sure I had it right. Then I got distracted. I'll try to get back to it.