Update minimum Go version to 1.12.

dev: build by go1.13 for release

fix golangci#529: exit with code 7 when error was logged

fix golangci#522: run misspell in text mode

Treat Go source files as plain text files by misspell: it allows detecting
issues in strings, variable names, etc. Also, it's the default mode of
a standalone misspell tool.

Also, implement richer and more stable auto-fix of misspell issues:
now it can fix multiple issues in one line.

docs: march updates

dev: add gosec test case

add gosec files

docs: remove extra bracket

don't crash when staticcheck panics

docs: update changelog