feat(assert): add optional custom label parameter to assert functions (#77)

Most assert functions now accept an optional trailing label parameter
that overrides the auto-derived test function name in failure output.

Examples:
  assert_same "expected" "$actual" "checking user name"
  assert_empty "$result" "result should be empty"
  assert_not_same "foo" "$val" "values must differ"

Affected functions in src/assert.sh:
  assert_same, assert_equals, assert_not_equals, assert_not_same,
  assert_empty, assert_not_empty,
  assert_less_than, assert_less_or_equal_than,
  assert_greater_than, assert_greater_or_equal_than,
  assert_contains_ignore_case,
  assert_string_not_starts_with,
  assert_string_matches_format, assert_string_not_matches_format

Variadic functions (assert_contains, assert_not_contains, assert_matches,
assert_not_matches, assert_string_starts_with, assert_string_ends_with,
assert_string_not_ends_with, assert_line_count) and exit-code functions
($3 already taken) do not support the label override.

Note: assert_files.sh and assert_folders.sh already supported custom
labels via their existing optional parameters.

Author: SauronBot