Add position checking to autocomplete

[isaactodo]

Changes

• Only the first word will be matched for command keywords
• Subsequent words will be matched for student fields only

Closes #113

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...seedu/address/logic/parser/AutocompleteParser.java	83.33%	0 Missing and 1 partial :warning:

Files with missing lines	Coverage Δ	Complexity Δ
...seedu/address/logic/parser/AutocompleteParser.java	91.17% <83.33%> (-0.58%)	47.00 <2.00> (+3.00)	:arrow_down: