Promote subscribe and add to all contexts

[qjqqyy]

Summary of changes

1. promotes subscribe to all contexts (closes #153)
2. rename feed context add to addindex (alias addi) prevents collision with 3
3. promotes add to all contexts for consistency with 1
4. select command has been lying in multiple (EntryBookSearchParser and EntryBookFeedsParser) parsers on top of being in the base EntryBookParser, so we standardise it by removing the SelectCommand reference in EntryBookParser. rationale is that select will behave differently in future versions in feeds context

while here improve some Parser tests, while here also add help message in bing command since it was missing.

[coveralls]

Pull Request Test Coverage Report for Build 832

• 8 of 11 (72.73%) changed or added relevant lines in 7 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+2.2%) to 85.04%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/main/java/seedu/address/logic/parser/AddIndexCommandParser.java	2	3	66.67%
src/main/java/seedu/address/logic/parser/EntryBookArchivesParser.java	0	1	0.0%
src/main/java/seedu/address/logic/parser/EntryBookParser.java	1	2	50.0%
<!--	Total:	8	11	72.73%	-->

Totals
Change from base Build 830:	2.2%
Covered Lines:	2143
Relevant Lines:	2520

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💛 - Coveralls