Add a configuration parameter for trimming whitespace on save

[iainh]

Prior to this change trailing whitespace is always remove when a file is saved. This can be expensive for very large files and in some cases may not be wanted by the user. Add a new configuration parameter trim_trailing_whitespace_on_save to allow the user to change the behaviour globally.

This change also moves the call to strip_trailing_whitespace() before the call to File::create reducing the amount of time that between when the file has been truncated and when the contents are written back.

[kevinmatthes]

Thank you for this Pull Request!

Just one question: did you test whether the assertion in zee/src/syntax/parse.rs, line 163, is still triggered when entering text after cropping whitespaces on saving?

[mcobzarenco]

@iainh Looks good, thanks for the changes!

I imagine eventually we want to be able to run mode-specific linters on save, so this option would have to be mode specific. But that's for the future.

This change also moves the call to strip_trailing_whitespace() before the call to File::create reducing the amount of time that between when the file has been truncated and when the contents are written back.

The one downside of this is that we trim the whitespace, even if we may not be able to create the file -- say for permissions reasons, io error etc.

@kevinmatthes On

Just one question: did you test whether the assertion in zee/src/syntax/parse.rs, line 163, is still triggered when entering text after cropping whitespaces on saving?

I think that should be fine. Btw, looking into the colour panic today -- sorry for the delay.