New truncate and truncateWords methods

New truncate and truncateWords methods, https://github.com/yiisoft/yii2/issues/2941
Update BaseStringHelper.php

Length check.
Update CHANGELOG.md
Update BaseStringHelper.php

Updated to use mb_* functions
Test case for new truncate methods.

Test case for new truncate methods.
Update BaseStringHelper.php
Update BaseStringHelper.php
Changelog update
setting writable folders in init-script

It is said, that these folders are handled already by composer.json, what is definitely true. But if I can't (or don't want to) call composer create-project command, I have to run post-create-project-cmd manually like `composer run-script post-create-project-cmd`. IMHO, it's consistent to make such preparations in init script.
no need to set writable folders for console
removed unnecessary lines regarding writable dirs
Lithuanian translation
Fixes #2906: Added support for using conditional comments for js and css files registered through asset bundles and Html helper
Edited intro, usage

Not sure about the “for example…” paragraph under “usage”. Doesn’t read
well currently.
Completed edits
new guide outline proposal
Fixes #3268: Fixed the bug that the schema name in a table name was not respected by `yii\db\mysql\Schema`
Merge pull request #3260 from adamaltman/master

Improved the advanced app template documentation.
fixed #3268
Merge branch 'master' of git://github.com/yiisoft/yii2