replacing bluebird with native promises

[cton]

Because the latest version of this library is incompatible with the angular version 8.3.19 I have replaced bluebird with native promises as proposed in #339. @Keyang please review the changes I've made and merge if everything is working.

[coveralls]

Pull Request Test Coverage Report for Build 215

• 18 of 18 (100.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.05%) to 95.409%

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Totals
Change from base Build 212:	0.05%
Covered Lines:	1322
Relevant Lines:	1354

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💛 - Coveralls

[jeremyrajan]

Not sure, if we need this or not. @Keyang thoughts?

[Keyang]

@jeremyrajan Thanks. Bluebird was used because it was fastest promise implementation (faster than native promise) at the time, especially for node 6. If we want to drop Bluebird we probably want to drop support of node 6 as well.
Alternatively, we could expose a package level config field for promise library and allow user to override the underline Promise implementation. Like require("csvtojson").Promise=Promise.
What would you think?

[jeremyrajan]

Ah ok! :)
I think a lot of providers have already stopped support for node 6. May be let look at 8+. LTS is at 12 now :D. And then we can get this PR to include native promises :)

[Jolg42]

It would be great to have this merged!

Node 10 is now in maintenance so older versions should definitely be dropped.
https://nodejs.org/en/about/releases/

[semla]

afaict rxjs can not be used with bluebird promises. Any chance of merging this?
https://stackoverflow.com/questions/65457823/merge-two-observables-in-angular-for-csv-to-json-conversion