Use predefined bucket name

[Illutax]

I was using influxql2flux for a while now and noticed an even better solution than #2 would be to have an input-field, where you could define your bucket name instead of <bucket_name>.

I've never wrote any react code, but I'll try to come up with a solution.

[Illutax]

Or use v.defaultBucket instead of "<bucket_name>" because it is possilbe to declare a default bucket in grafana which can be accessed via v.defaultBucket.