Or (A|B)

[`A|B`](https://docs.python.org/3.7/library/re.html#index-13)

**Normal regex**:
```py
from mre import Regex

# Example of how it can work
print(Regex('Test').or(Regex('test')))

# Output: Test|test
# Regex without |: Testtest
```

**Group**:
```py
from mre import Regex, Group

# Example of how it can work
print(Group('Test').or(Regex('test')))

# Output: (Test|test)
# Regex without |: (Testtest)
```

----------

**Attention** to Set behavior:
```py
from mre import Regex, Set

# Example of how it can work
print(Set('Test').or(Regex('test')))

# Output: [Test|test]
# Regex without |: [Testtest]
```
Operator `or` it makes no sense to have the default behavior in this case.