Add linear calibration sensor filter (#454)

* Add linear calibrate filter

* Remove filter_nan

* Add test

esphome/config_validation.py

@@ -66,7 +66,7 @@ def valid_name(value):
     for c in value:
         if c not in ALLOWED_NAME_CHARS:
             raise vol.Invalid(u"'{}' is an invalid character for names. Valid characters are: {}"
-                              u"".format(c, ALLOWED_NAME_CHARS))
+                              u" (lowercase, no spaces)".format(c, ALLOWED_NAME_CHARS))
     return value