Change to feature version bump

Code using the new argument should depend on this version.

CHANGELOG.md

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 26f4c5c..a205299 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,4 +1,4 @@
-## 2.5.1-wip
+## 2.6.0-wip
 
 * Added source argument when throwing a `ArgParserException`.
 * Fix inconsistent `FormatException` messages

pubspec.yaml

diff --git a/pubspec.yaml b/pubspec.yaml
index 2b70992..8e59181 100644
--- a/pubspec.yaml
+++ b/pubspec.yaml
@@ -1,5 +1,5 @@
 name: args
-version: 2.5.1-wip
+version: 2.6.0-wip
 description: >-
   Library for defining parsers for parsing raw command-line arguments into a set
   of options and values using GNU and POSIX style options.