Merge pull request mna#135 from mavolin/fix/134

Fix Backslash Escapes Not Being Properly Handled

Merge pull request mna#131 from mna/cleanup-linting

Update linting to golangci-lint

Calculate litMatcher.want at codegen time (mna#95)

parseLitMatcher() was calling strconv.AppendQuote() each time, which my
profiler discovered was costing a measurably large amount of runtime
during parsing within dhall-golang.

This commit calculates litMatcher.want at codegen time so that we don't
have to compute the `want` value each time we parse with the litMatcher.

update README for releases and new maintainer