Using x in prerelease causes error

[danielhelfand]

Given the version 0.1.0-x, ParseRange returns the following error:

Could not get version from string: "<"

Replacing the x with another character fixes the problem (e.g. 0.1.0-y).

This appears to be related to x being used to denote wildcards as implemented in #27:

semver/v4/range.go

Line 330 in 4487282

if strings.Contains(ap, "x") {

This can be reproduced with the following test:

func TestExpandWildcardVersion(t *testing.T) {
	tests := []struct {
		i [][]string
		o [][]string
	}{
		{[][]string{{"foox"}}, nil},

                // FAILING TEST
		{[][]string{{"0.1.0-x"}}, [][]string{{"0.1.0-x"}}},


		{[][]string{{">=1.2.x"}}, [][]string{{">=1.2.0"}}},
		{[][]string{{"<=1.2.x"}}, [][]string{{"<1.3.0"}}},
		{[][]string{{">1.2.x"}}, [][]string{{">=1.3.0"}}},
		{[][]string{{"<1.2.x"}}, [][]string{{"<1.2.0"}}},
		{[][]string{{"!=1.2.x"}}, [][]string{{"<1.2.0", ">=1.3.0"}}},
		{[][]string{{">=1.x"}}, [][]string{{">=1.0.0"}}},
		{[][]string{{"<=1.x"}}, [][]string{{"<2.0.0"}}},
		{[][]string{{">1.x"}}, [][]string{{">=2.0.0"}}},
		{[][]string{{"<1.x"}}, [][]string{{"<1.0.0"}}},
		{[][]string{{"!=1.x"}}, [][]string{{"<1.0.0", ">=2.0.0"}}},
		{[][]string{{"1.2.x"}}, [][]string{{">=1.2.0", "<1.3.0"}}},
		{[][]string{{"1.x"}}, [][]string{{">=1.0.0", "<2.0.0"}}},

	}

	for _, tc := range tests {
		o, _ := expandWildcardVersion(tc.i)
		if !reflect.DeepEqual(tc.o, o) {
			t.Errorf("Invalid for case %q: Expected %q, got: %q", tc.i, tc.o, o)
		}
	}
}

Result of this test is: Invalid for case [["0.1.0-x"]]: Expected [["0.1.0-x"]], got: [[">=0.1.0-x" "<"]]

At the very least, it would be helpful to call out this corner case in the docs if fixing this would be challenging due to breaking changes.