Fixed the incorrect behavior of `insert()`, added `insert_key_value()` api. Updated a lot of more detailed documents.

See #274, release log:

• add284f by @yegor256: Merge pull request #274 from o...
• 220b80f by @owtotwo: fix: use asterisk instead of d...
• 7b56064 by @owtotwo: fix: use feature instead of li...
• 675f324 by @owtotwo: feat: use nightly Rust in docs...
• c32d44f by @owtotwo: feat: add a new api `with_capa...
• ffc9d3e by @owtotwo: fix: insert() api should not...
• bfe1874 by @owtotwo: doc: for clear()
• b680046 by @owtotwo: refactor: rewrite the Display ...
• e44d6ae by @owtotwo: refactor: impl clone() by it...
• 8d1c355 by @owtotwo: doc: Improved constructor rela...
• 8e29d8f by @owtotwo: readme: add cmd line about doc...
• bc17357 by @owtotwo: doc: add crate attributes abou...
• 171c506 by @rultor: Merge branch '__rultor'
• ea496d9 by @yegor256: Merge pull request #272 from o...
• 84f721e by @owtotwo: fix: the doc test
• 45768f3 by @owtotwo: feat: add Sub trait for `Set...
• 9022a23 by @owtotwo: markdown: fix lint for long li...
• 9ee2825 by @owtotwo: readme: add the MSRV badge to ...
• e0cc5a1 by @owtotwo: manifest: set MSRV to Rust 1.7...
• 5385cdb by @yegor256: Merge pull request #271 from o...
• and 5 more...

Released by Rultor 2.0-SNAPSHOT, see build log