Allow the arguments to ag itself to be overriden, if a prefix argument
is used.

Fixes #36.
Documenting overriding ag flags.
Mark ag-project-at-point as obsolete.
Clarify in docstring that we always return a string.
Changing ag-project-regexp to default to an escaped regexp of the symbol
at point.

This makes `ag-regexp-project-at-point` at point redundant. The other
FOO-at-point commands were removed (without breaking compatibility)
several versions ago. `ag-regexp-project-at-point` was also
inconsistently named.
Updating changelog for 0.40.
Merge branch 'master' into travis_unit_tests