Enable `smartindent` option for default configuration

[iharkatkavets]

This PR enables a pretty useful option that probably exists in most IDE's by default - indentations. If you see if that can be useful to be included in basic configuration, please merge my PR. If not, I think it has to be at least mentioned(and commented) in the basic configuration.

[iton0]

What's the purpose of vim-sleuth if this in an option in vim

[feoh]

Yup thanks for your contribution but like @iton0 said we have vim-sleuth for this.