Allow slow concat time to be configurable

[balos1]

This is related to #36 #38. Currently, the only way to suppress the message:

[WARN] (broccoli-uglify-sourcemap) Minifying: 'assets/somefile.js' took: 21087ms (more than 20,000ms)

is by using the --silent option, which also suppresses some other messages which I don't want to suppress. I propose that we allow the "slow" concat time to be configurable (it is hard coded at 20000ms right now). Then users can suppress this message only by increasing the time.

[quaertym]

I wonder why it is taking so long. I upgraded from broccoli-uglify-js to this and uglify time increased from 9000ms to 24000ms. I had to revert to broccoli-uglify-js since this has slowed down development drastically.

[iballack]

Any recent update on this issue?

[anpenava]

still nothing :)?

[rwjblue]

Seems like a great pull request to make 😸