lint: Add spell check linter (codespell)

.travis/lint_04_install.sh

@@ -6,4 +6,5 @@
 
 export LC_ALL=C
 
+travis_retry pip install codespell
 travis_retry pip install flake8

test/lint/lint-spelling.ignore-words.txt

@@ -2,4 +2,5 @@ cas
 hights
 mor
 objext
+unselect
 useable

test/lint/lint-spelling.sh

@@ -0,0 +1,15 @@
+#!/usr/bin/env bash
+#
+# Copyright (c) 2018 The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+#
+# Warn in case of spelling errors.
+# Note: Will exit successfully regardless of spelling errors.
+
+export LC_ALL=C
+
+IGNORE_WORDS_FILE=test/lint/lint-spelling.ignore-words.txt
+if ! codespell --check-filenames --disable-colors --quiet-level=7 --ignore-words=${IGNORE_WORDS_FILE} $(git ls-files -- ":(exclude)build-aux/m4/" ":(exclude)contrib/seeds/*.txt" ":(exclude)depends/" ":(exclude)doc/release-notes/" ":(exclude)src/leveldb/" ":(exclude)src/qt/locale/" ":(exclude)src/secp256k1/" ":(exclude)src/univalue/"); then
+    echo "^ Warning: codespell identified likely spelling errors. Any false positives? Add them to the list of ignored words in ${IGNORE_WORDS_FILE}"
+fi