Fix multiple logger handlers when custom logger is used

When `logger` function is used, it adds logger handler
after the static file handler. This causes 404 errors for
static files. Default logger is also added this makes two
logger handlers per every request.

Reproducer:

```crystal
require "kemal"

public_folder "./public"

get "/hello" do |env|
  "Hello!"
end

class MyCustomLogger < Kemal::BaseLogHandler
  def call(context)
    puts "I'm logging some custom stuff here."
    call_next(context) # => This calls the next handler
  end

  # This is used from `log` method.
  def write(message)
    STDERR.puts message # => Logs the output to STDERR
  end
end

\# This works
\# Kemal.config.logger = MyCustomLogger.new

\# This fails while serving static files
logger MyCustomLogger.new

Kemal.run

```

With this PR, `logger` function only updates the `Kemal.config`
and `Kemal.run` will take care of adding logger handler.

This PR also fixes the warning when tried to run the example code
for adding custom logger.

```
 10 | def call(env)
               ^--
Warning: positional parameter 'env' corresponds to
parameter 'context' of the overridden method
Kemal::BaseLogHandler#call(context : HTTP::Server::Context),
which has a different name and may affect named argument passing
```

Signed-off-by: Aravinda Vishwanathapura <mail@aravindavk.in>

src/kemal/helpers/helpers.cr

@@ -48,13 +48,13 @@ end
 # This is used to replace the built-in `Kemal::LogHandler` with a custom logger.
 #
 # A custom logger must inherit from `Kemal::BaseLogHandler` and must implement
-# `call(env)`, `write(message)` methods.
+# `call(context)`, `write(message)` methods.
 #
 # ```
 # class MyCustomLogger < Kemal::BaseLogHandler
-#   def call(env)
+#   def call(context)
 #     puts "I'm logging some custom stuff here."
-#     call_next(env) # => This calls the next handler
+#     call_next(context) # => This calls the next handler
 #   end
 #
 #   # This is used from `log` method.
@@ -71,7 +71,6 @@ end
 # ```
 def logger(logger : Kemal::BaseLogHandler)
   Kemal.config.logger = logger
-  Kemal.config.add_handler logger
 end
 
 # Enables / Disables static file serving.