Add support for filtering based on property exclusion.

For example, allow commands like:

  # exclude issues from context "abc"
  pit list -C abc

  # exclude issues delegated to John Doe
  pit list -P "delgated-to:John Doe"

src/pitpkg/private/libpit.nim

@@ -24,6 +24,7 @@ type
     fullMatch*, summaryMatch*: Option[Regex]
     hasTags*: seq[string]
     properties*: TableRef[string, string]
+    exclProperties*: TableRef[string, seq[string]]
 
   PitConfig* = ref object
     tasksDir*: string
@@ -114,7 +115,8 @@ proc initFilter*(): IssueFilter =
     fullMatch: none(Regex),
     summaryMatch: none(Regex),
     hasTags: @[],
-    properties: newTable[string, string]())
+    properties: newTable[string, string](),
+    exclProperties: newTable[string,seq[string]]())
 
 proc propsFilter*(props: TableRef[string, string]): IssueFilter =
   if isNil(props):
@@ -347,6 +349,12 @@ proc filter*(issues: seq[Issue], filter: IssueFilter): seq[Issue] =
   for k,v in filter.properties:
     result = result.filterIt(it.hasProp(k) and it[k] == v)
 
+  for k,v in filter.exclProperties:
+    result = result.filter(proc (iss: Issue): bool =
+      not iss.hasProp(k) or
+      not v.anyIt(it == iss[k])
+    )
+
   if filter.completedRange.isSome:
     let range = filter.completedRange.get
     result = result.filterIt(