codegen: Lambda improvements (#1476)

* Use #line directives in generated C++ code for lambdas

The #line directive in gcc is meant specifically for pieces of imported
code included in generated code, exactly what happens with lambdas in
the yaml files: https://gcc.gnu.org/onlinedocs/cpp/Line-Control.html

With this change, if I add the following at line 165 of kithen.yaml:
    - lambda: undefined_var == 5;

then "$ esphome kitchen.yaml compile" shows the following:

INFO Reading configuration kitchen.yaml...
INFO Generating C++ source...
INFO Compiling app...
INFO Running:  platformio run -d kitchen
<...>
Compiling .pioenvs/kitchen/src/main.cpp.o
kitchen.yaml: In lambda function:
kitchen.yaml:165:7: error: 'undefined_var' was not declared in this scope
*** [.pioenvs/kitchen/src/main.cpp.o] Error 1
== [FAILED] Took 2.37 seconds ==

* Silence gcc warning on multiline macros in lambdas

When the \ is used at the end of the C++ source in a lambda (line
continuation, often used in preprocessor macros), esphome will copy that
into main.cpp once as code and once as a // commment.  gcc will complain
about the multiline commment:

Compiling .pioenvs/kitchen/src/main.cpp.o
kitchen.yaml:640:3: warning: multi-line comment [-Wcomment]

Try to replace the \ with a "<cont>" for lack of a better idea.

esphome/yaml_util.py

@@ -11,7 +11,8 @@ import yaml.constructor
 
 from esphome import core
 from esphome.config_helpers import read_config_file
-from esphome.core import EsphomeError, IPAddress, Lambda, MACAddress, TimePeriod, DocumentRange
+from esphome.core import EsphomeError, IPAddress, Lambda, MACAddress, TimePeriod, \
+    DocumentRange, DocumentLocation
 from esphome.helpers import add_class_to_obj
 from esphome.util import OrderedDict, filter_yaml_files
 
@@ -30,9 +31,16 @@ class ESPHomeDataBase:
     def esp_range(self):
         return getattr(self, '_esp_range', None)
 
+    @property
+    def content_offset(self):
+        return getattr(self, '_content_offset', 0)
+
     def from_node(self, node):
         # pylint: disable=attribute-defined-outside-init
         self._esp_range = DocumentRange.from_marks(node.start_mark, node.end_mark)
+        if isinstance(node, yaml.ScalarNode):
+            if node.style is not None and node.style in '|>':
+                self._content_offset = 1
 
 
 class ESPForceValue:
@@ -257,7 +265,10 @@ class ESPHomeLoader(yaml.SafeLoader):  # pylint: disable=too-many-ancestors
 
     @_add_data_ref
     def construct_lambda(self, node):
-        return Lambda(str(node.value))
+        start_mark = DocumentLocation.from_mark(node.start_mark)
+        if node.style is not None and node.style in '|>':
+            start_mark.line += 1
+        return Lambda(str(node.value), start_mark)
 
     @_add_data_ref
     def construct_force(self, node):