Improve yaml string highlighting

Fixes #783

runtime/syntax/ledger.yaml

@@ -1,7 +1,7 @@
 filetype: ledger
 
 detect: 
-    filename: "(^|\\.|/)ledger|ldgr|beancount|bnct$"
+    filename: "(^|\\.|/)(ledger|ldgr|beancount|bnct)$"
 
 rules:
     - special: "^([0-9]{4}(/|-)[0-9]{2}(/|-)[0-9]{2}|[=~]) .*"

runtime/syntax/yaml.yaml

@@ -15,16 +15,16 @@ rules:
     - special:  "(^---|^\\.\\.\\.|^%YAML|^%TAG)"
 
     - constant.string:
-        start: "\""
+        start: "(^| )\""
         end: "\""
         skip: "\\\\."
         rules:
             - constant.specialChar: "\\\\."
 
     - constant.string:
-        start: "'"
+        start: "(^| )'"
         end: "'"
-        skip: "\\\\."
+        skip: "(\\\\.)|('')"
         rules:
             - constant.specialChar: "\\\\."