Add support for new CLDR plural operands e and c.

[devversion]

The cldr package currently comes with CLDR 39 but the actual
plural expressions cannot be properly extracted/generated.

For example, the french plural uses the new e operand, but
the parser does not know about such. This commit adds support
for the new operands.

@papandreou When you get a chance, I'd appreciate your review on this. As mentioned,
currently extracting the plurals from CLDR 39 is not working in all cases. e.g.

(node:19684) UnhandledPromiseRejectionWarning: SyntaxError: Expected " not ", "f", "i", "n", "t", "v", "w", [0-9], or [\-+] but "e" found.
    at peg$buildStructuredError (eval at compile (C:\users\paul\_bazel_paul\gsqtgr5s\execroot\angular\node_modules\pegjs\lib\compiler\index.js:67:29), <anonymous>:343:14)
    at Object.peg$parse [as parse] (eval at compile (C:\users\paul\_bazel_paul\gsqtgr5s\execroot\angular\node_modules\pegjs\lib\compiler\index.js:67:29), <anonymous>:1164:13)
    at new CldrPluralRule (C:\users\paul\_bazel_paul\gsqtgr5s\execroot\angular\node_modules\cldr\lib\CldrPluralRule.js:209:30)
    at CldrPluralRuleSet.addRule (C:\users\paul\_bazel_paul\gsqtgr5s\execroot\angular\node_modules\cldr\lib\CldrPluralRuleSet.js:20:24)

[coveralls]

Coverage remained the same at 76.008% when pulling c0305d1 on devversion:master into f3c9fd4 on papandreou:master.

[papandreou]

Thanks for looking into this and for providing a high quality PR 💪

Released the fix in 6.1.1

[devversion]

Thanks for the quick review and release!