Add tests for parsers

[thomastanck]

Increase coverage by adding tests for parsers.

Summary of changes:

• Made ParserUtil abstract
• Tests added for:
  • SubscribeCommandParser
  • UnsubscribeCommandParser
  • AddIndexCommandParser
  • BingWebSearchCommandParser
  • GoogleNewsCommandParser
  • DeleteArchiveEntryCommandParser
  • RefreshEntryCommandParser
  • RefreshFeedCommandParser
• Tests updated for:
  • ParserUtil#parseIndex
  • EntryBookArchivesParserTest
  • EntryBookFeedsParserTest
  • EntryBookListParserTest
  • EntryBookSearchParserTest

[coveralls]

Pull Request Test Coverage Report for Build 917

• 0 of 1 (0.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+1.9%) to 86.682%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/main/java/seedu/address/logic/parser/ParserUtil.java	0	1	0.0%
<!--	Total:	0	1	0.0%	-->

Totals
Change from base Build 910:	1.9%
Covered Lines:	2239
Relevant Lines:	2583

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💛 - Coveralls