add ability to run svgr in sync mode

[lifeiscontent]

@neoziro I was thinking of creating a svgr/macro to run svgr as a macro but realize when writing a macro the code needs to run synchronously. Do you think we could make a sync option for svgr to add this functionality?

[gregberge]

I think it is already done https://github.com/evenchange4/svgr.macro. And for the sync mode, the answer is no. I prefer to keep it asynchronous.

[lifeiscontent]

@neoziro I understand that the preferred method is async, but sometimes there are reasons why you would run in sync mode, e.g. this macro. the macro currently is using a execSync function from child process, it'd be nice to have an API so you don't have to run a separate process. Also, it'd be cool to support a glob pattern.

TLDR, the reason why I'm asking is @evenchange4 's current implementation is incredibly slow do to the way the code is executed.

[gregberge]

Yes, but I don't know if it is even possible to run it synchronously. Let's reopen it.

[gregberge]

Follow up in #185.