csv: Add docs and integration test for & operator

hledger-lib/hledger_csv.m4.md

@@ -566,7 +566,15 @@ CSVFIELD is a percent sign followed by the field's name or column number, like `
 
 A single matcher can be written on the same line as the "if";
 or multiple matchers can be written on the following lines, non-indented.
-Multiple matchers are OR'd (any one of them can match).
+Multiple matchers are OR'd (any one of them can match), unless one includes 
+an `&` symbol, in which case it is combined with the previous matcher.
+
+```rules
+if
+MATCHER
+& MATCHER
+ RULE
+```
 
 After the patterns there should be one or more rules to apply, all
 indented by at least one space. Three kinds of rule are allowed in

tests/csv.test

@@ -882,6 +882,30 @@ start of conditional block found, but no assignment rules afterward
 )
 >=1
 
+# 44. handle conditions with & operator
+<
+10/2009/09,Flubber Co,50
+10/2009/09,Blubber Co,50
+
+RULES
+fields date, description, amount
+date-format %d/%Y/%m
+currency $
+account1 assets:myacct
+if Flubber
+& %amount 50
+  account2 acct
+$  ./csvtest.sh
+2009-09-10 Flubber Co
+    assets:myacct             $50
+    acct                     $-50
+
+2009-09-10 Blubber Co
+    assets:myacct              $50
+    income:unknown            $-50
+
+>=0
+
 
 ## . 
 #<