Added signature separator option plus some refactoring

[AckslD]

closes #113

I started adding an option to change the default '--' to separate the content and signature of a message. While doing so I also merge some common code for the build_xxx functions.

[soywod]

I like the way you refactored, it looks cleaner to me!

[soywod]

@AckslD In fact, the double dash seems to be a standard. In the .signature file, the dashes should not be included because they are added automatically. I tried with emacs and thunderbird and both have the same behaviour. I think we should reconsider this PR to align the tool with standards.

I propose sth close to what you originally proposed: we introduce a new boolean signature-delimiter. If true (the default) then we show the delimiter before the signature. If not then only the signature is shown. Those who want a custom delimiter can disable the signature-delimiter and include it inside their signature. What do you think?

[AckslD]

@soywod I see, yeah that make sense to me 👍

[soywod]

@AckslD I pushed the changes (a6b30b7). I introduced signature-delimiter as on optional string instead of a boolean, I thought it would be more UX. In fact it really looks like your initial PR... sorry for all those changes.

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It's been a while I did not see you here, I hope your are doing well!

[AckslD]

Nice @soywod! :)

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I started a new job so been quite busy with that :) Hope you're doing well also!