Update default value of unit_context argument of uparse() method

[michikawa07]

Nevertheless, expanded pkg of Unitful.jl (i.e., UnitfulAtomic.jl) execute Unitful.register(), Unitful.uparse did not recognize these pkgs automatically (due to the defaut value of unit_context arg is Unitful only).
This commit modified this issue.

The following example is picked up from runtest.jl

# Test that the @u_str macro will not find units in modules which are
# not loaded before the u_str invocation.
module FooUnits
    using Unitful
    @unit foo "foo" MyFoo 1u"m" false
    Unitful.register(FooUnits)
end
# ...
# Test that u_str works when FooUnits is correctly loaded.
module DoesUseFooUnits
    using Unitful, ..FooUnits
    foo() = 1u"foo"
end
@test DoesUseFooUnits.foo() === 1u"foo"

# Tests for unit extension modules in unit parsing
@test_throws ArgumentError uparse("foo", unit_context=Unitful)
################################ Here is the effect of this PR
@test uparse("foo") === u"foo" # This work, because default `unit_context` is now `vcat(Unitful, Unitful.unitmodules)`
################################
@test uparse("foo", unit_context=FooUnits) === u"foo"
@test uparse("foo", unit_context=[Unitful, FooUnits]) === u"foo"
@test uparse("foo", unit_context=[FooUnits, Unitful]) === u"foo"