syntax: support `elif` clauses for alternative `if` syntax

In the alternative `if` syntax mode, support a specific `elif` keyword instead
of requiring an `else` branch followed by a disjunct `if` statement.

The advantage is that templates do not require error-prone redundant `endif`
keywords in else-if ladders.

After this change, the following example:

    {% if (...): %}
    One condition
    {% else if (...): %}
    Another condition
    {% else if (...): %}
    A third condition
    {% else %}
    Final condition
    {% endif; endif; endif %}

... can be simplified into:

    {% if (...): %}
    One condition
    {% elif (...): %}
    Another condition
    {% elif (...): %}
    A third condition
    {% else %}
    Final condition
    {% endif %}

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

1 files changed, 16 insertions, 0 deletions

diff --git a/tests/00_syntax/18_if_condition b/tests/00_syntax/18_if_condition
index 9601f11..9e02767 100644
--- a/tests/00_syntax/18_if_condition
+++ b/tests/00_syntax/18_if_condition
@@ -22,6 +22,10 @@ An if-condition using the alternative syntax:
 Variable x has another value.
 
 
+An if-condition using the special "elif" keyword in alternative syntax mode:
+Variable x was set to five.
+
+
 Ternary expressions function similar to if/else statements but
 only allow for a single expression in the true and false branches:
 Variable x is one
@@ -93,6 +97,18 @@ Variable x has another value.
 {% endif %}
 
 
+An if-condition using the special "elif" keyword in alternative syntax mode:
+{% if (x == 0): -%}
+Variable x was set to zero.
+{% elif (x == 1): -%}
+Variable x was set to one.
+{% elif (x == 5): -%}
+Variable x was set to five.
+{% else -%}
+Variable x has another value.
+{% endif %}
+
+
 Ternary expressions function similar to if/else statements but
 only allow for a single expression in the true and false branches:
 {%