Add support for merging patterns

[qzb]

Despite of what README states, regular expressions can be merged using zero-length assertions:
https://www.regular-expressions.info/lookaround.html

[mokkabonna]

Thanks!

[mokkabonna]

This turned out to not work as expected, see #44

For example this works as expected:

/(?=1)(?=\d)/.test('1') // true

This does not:

/(?=1)(?=2)/.test('12') // false

This however works as expected:

/(?=.*1)(?=.*2)/.test('12') // true