Add support for .lintignore.

Lint-ing all of Weave Net is currently not desirable because:

* it would create conflicts on many branches,
* it would pollute the git history,
* etc.

and because `lint` currently lints all files except `./vendor`, this breaks Weave Net's build.
As a result, Weave Net is not using the latest commit in `build-tools`'s `master` branch, which is also source of problems:

* it limits standardisation of projects across Weaveworks,
* it limits the ability to use the latest features of `build-tools`,
* it complexifies merging changes in `build-tools` (e.g. #53 and weaveworks/weave#2694 for weaveworks/weave#2647),
* etc.

This change adds the ability to optionally process a `.lintignore` file, which can contain any number of `GLOB`s then used to filter out files one does not want to lint, a-la `.gitignore`.
This, coupled with a `.lintignore` file in Weave Net will then enable Weave Net to use the latest version of `build-tools`.

lint

@@ -171,6 +171,25 @@ lint_files() {
     exit $lint_result
 }
 
+filter_out() {
+    local globs="$1"
+    if [ -n "$globs" -a -r "$globs" ]; then
+        # Files to debug the regular expressions, as they get quite hairy:
+        local dir=${CIRCLE_ARTIFACTS:-/tmp}
+        local files_in=$(mktemp $dir/linted_files_in_XXX.txt)
+        local files_out=$(mktemp $dir/linted_files_out_XXX.txt)
+        # Globs blacklist found: filter out stdin before passing it to stdout:
+        while read -r filename; do
+            # Remove comments. Remove empty lines. Expand directories (e.g. "bin/" -> "bin/.*"). And then regexify globs.
+            # These regexes are then passed to grep to filter out matching elements from stdin:
+            echo "${filename}" | tee -a "$files_in" | grep -vEf <(sed '/^#.*$/d ; /^\s*$/d ; s/^\(.*\/\)$/\1*/g ; s/\([.|]\)/\\\1/g ; s/\?/./g ; s/\*/.*/g ; s/\(.*\)/\^\1\$/g' "$globs") | tee -a "$files_out"
+        done
+    else
+        # No globs blacklist was provided: simply propagate stdin to stdout:
+        while read -r filename; do echo "${filename}"; done
+    fi
+}
+
 list_files() {
     if [ $# -gt 0 ]; then
         git ls-files --exclude-standard | grep -vE '(^|/)vendor/'
@@ -179,4 +198,4 @@ list_files() {
     fi
 }
 
-list_files "$@" | lint_files
+list_files "$@" | filter_out .lintignore | lint_files