Generate better comments for require package

[Neokil]

Summary

The comments for the require package were just copied over from the assert package when generating the functions. This could lead to confusion because

1. The code-examples were showing examples using the assert package instead of the require package
2. The function-documentation was not mentioning that the functions were calling t.FailNow() which is some critical information when using this package.

Changes

1. Added a "replace" function to the template
2. require.go.tmpl now replaces the package name accordingly
3. require.go.tmpl now adds another line to the function-docs explaining the difference to the assert-function

Motivation

When new developers are coming in they most likely did not go through the github-documentation for every package used in the application. But what they do is read the function-docs when they go through code to understand what it is doing. In case of test-cases using both require and assert it gets confusing because the function docs for the require package are referencing the assert package and don't really document the actual function.

[dolmen]

I've just merged #1607, so please rebase and run codegen.

[hendrywiranto]

this will close https://github.com/stretchr/testify/issues/1609 too

[Neokil]

somehow I cannot add reviewers the to the PR, but maybe @boyan-soubachov, @dolmen, @MovieStoreGuy, @arjunmahishi or @brackendawson can one of you have a look?