feat: else support for boolean helpers

[exoego]

This PR makes boolean helpers (eq, and, etc) can be used with else.

{{#eq foo bar}}
OK
{{else}}
KO
{{/eq}}

// this still works
{{eq foo bar}}

It can be considered as a shorthand for {{#if (eq a b)}}SUCCESS{{else}}FAIL{{/if}}.

I need this to improve inter-operability with handlerbars.java's ConditionalHelpers

[coveralls]

coverage: 83.855% (-0.2%) from 84.017%
when pulling f71657c on exoego:conditional-block
into cda7412 on sunng87:master.

[sunng87]

LGTM

[sunng87]

@exoego Thank you!