Improve multi-line expressions

Resolves #12

This commit attempts to improve support for multi-line expressions.
If certain keywords/operators are detected at beginning of a line,
treat them as expression continuation rather than custom function
call statement.

Syntaxes/Liquid.sublime-syntax

@@ -167,6 +167,8 @@ contexts:
     - include: liquid-conditionals
     - include: liquid-iterations
     - include: liquid-functions
+    - include: liquid-filters
+    - include: liquid-expression-continuation
     - include: liquid-other
 
   liquid-expression:
@@ -441,6 +443,32 @@ contexts:
     - match: \bby\b
       scope: keyword.operator.liquid
 
+  liquid-expression-continuation:
+    # hack to handle multi-line expressions
+    #  If first token of a new line matches one of the following patterns
+    #  it is most likely a continuation of an expression.
+    - match: ':'
+      scope: punctuation.separator.key-value.liquid
+      push: liquid-expression
+    - match: ','
+      scope: punctuation.separator.sequence.liquid
+      push: liquid-expression
+    - match: =
+      scope: keyword.operator.assignment.liquid
+      push: liquid-expression
+    - match: (?:!=|==|<=|>=|<|>)
+      scope: keyword.operator.comparison.liquid
+      push: liquid-expression
+    - match: \bas\b
+      scope: keyword.operator.assignment.liquid
+      push: liquid-expression
+    - match: \b(?:and|or|not|contains)\b
+      scope: keyword.operator.logical.liquid
+      push: liquid-expression
+    - match: \bby\b
+      scope: keyword.operator.liquid
+      push: liquid-expression
+
 ###[ LIQUID LITERALS ]########################################################
 
   liquid-constants:

tests/syntax_test_liquid.liquid.html

@@ -77,6 +77,23 @@
 |                           ^^^^^^^ meta.string.liquid string.quoted.double.liquid
 |                                   ^^ punctuation.section.embedded.end.liquid
 
+{%- if foo != blank
+  and bar != ''
+|^^^^^^^^^^^^^^ meta.embedded.liquid source.liquid meta.statement.liquid
+| ^^^ keyword.operator.logical.liquid
+|     ^^^ variable.other.liquid
+|         ^^ keyword.operator.comparison.liquid
+|            ^^ meta.string.liquid string.quoted.single.liquid
+  or baz
+|^^^^^^^^ meta.embedded.liquid source.liquid meta.statement.liquid
+| ^^ keyword.operator.logical.liquid
+|    ^^^ variable.other.liquid
+  == true
+|^^^^^^^^ meta.embedded.liquid source.liquid meta.statement.liquid
+| ^^ keyword.operator.assignment.liquid
+|    ^^^^ constant.language.boolean.liquid
+-%}
+
 {% case handle %}
 |^^^^^^^^^^^^^^^^ meta.embedded.liquid source.liquid meta.statement.liquid
 |  ^^^^ keyword.control.conditional.case.liquid