Add more tests of placeholder flags and simplify its logic (#2624)

* [tests] Test fzf's placeholders and escaping on practical commands

This tests some reasonable commands in fzf's templates (for commands,
previews, rebinds etc.), how are those commands escaped (backslashes,
double quotes), and documents if the output is executable in cmd.exe.
Both on Unix and Windows.

* [tests] Add testing of placeholder parsing and matching

Adds tests and bit of docs for the curly brackets placeholders in fzf's
template strings. Also tests the "placeholder" regex.

* [tests] Add more test cases of replacing placeholders focused on flags

Replacing placeholders in templates is already tested, this adds tests
that focus more on the parameters of placeholders - e.g. flags, token
ranges.

There is at least one test for each flag, not all combinations are
tested though.

* [refactoring] Split OS-specific function quoteEntry() to corresponding source file

This is minor refactoring, and also the function's test was made
crossplatform.

* [refactoring] Simplify replacePlaceholder function

Should be equivalent to the original, but has simpler structure.

src/terminal_unix.go

@@ -5,6 +5,7 @@ package fzf
 import (
 	"os"
 	"os/signal"
+	"strings"
 	"syscall"
 )
 
@@ -19,3 +20,7 @@ func notifyStop(p *os.Process) {
 func notifyOnCont(resizeChan chan<- os.Signal) {
 	signal.Notify(resizeChan, syscall.SIGCONT)
 }
+
+func quoteEntry(entry string) string {
+	return "'" + strings.Replace(entry, "'", "'\\''", -1) + "'"
+}