`renderSectionContainer`

[OliverJAsh]

As originally suggested in https://github.com/moroshko/react-autosuggest/issues/358

For context, I needed this to build the following design:

[gallantdon]

Can this be merged? This would be extremely useful.

[frederik-oestergaard]

Also need this for a very customized autosuggestion popup. Currently what we have to do is very hacky and complex considering the simplicity of the problem. All we need to do is essentially render one section as flex row instead of flex column based on the type of section, but without this renderSectionContainer there is no simple way of achieving that