Fix title splitting logic to account for both newlines and periods

[eshrh]

This PR changes the logic for separating the title from the rest of the entry. This is intended to fix #945.

It's important to note that we can't just remove all sentence terminal checking, because then normal command-entry breaks. When a user enters an entry at the command line, they obviously can't naturally newline, so we still have to break the sentence by terminals. However, if there is a newline, then we can infer that that was where the user truly wanted the title to end because they were writing in a full text editor.

Because of this, it is impossible to fully fix #945, but it is possible to fix it if there is a newline character following the sentence.

Here are some cases:

User:
jrnl I did it! I won the competition. I am so happy
Result:

2020-05-19 16:15 I did it!
| I won the competition

This is no different from earlier

However, if the user in a text editor writes:
I did it! I won the competition.
I am so happy
The result is now

2020-05-19 16:17 I did it! I won the competition.
| I am so happy

This is different

This fixes #945 without messing up anything else.

Checklist

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[eshrh]

I lost track of the correct order of "",text when i was switching it around. aeafcd3 was supposed to fix this but it looks like it just made the mistake...

[wren]

I think this is a good compromise for the title splitting. Thank you!

Can you please add some tests for the changed functionality?

[eshrh]

I added one test for multi-line entries with punctuation and one test for single-line entries with punctuation.

[wren]

Thank you!