Merge commit from fork

* Remove strings.Split and add parseToken function

* review and add tests

Backporting 0951d18 to v4

Merge commit from fork

* Initial draft of adding text to ParseWithClaims

* Adjusted example and referring to the example in Parse functions

* Backporting logic from v5

* Added a test

Release v5.2.1

Release v5.2.0

Release v5.1.0

enable jwt.ParsePublicKeyFromPEM to parse PKCS1 Public Key (golang-jw…

…t#120)

Adjusting the error checking example (golang-jwt#270)

This PR adjusts the error checking example so that a check for an invalid signature is also included.

See discussion in golang-jwt#143

`v5` Pre-Release (golang-jwt#234)

Co-authored-by: Micah Parks <66095735+MicahParks@users.noreply.github.com>
Co-authored-by: Michael Fridman <mf192@icloud.com>

Allow strict base64 decoding (golang-jwt#259)

By default base64 decoder works in non-strict mode which
allows tweaking signatures having padding without failing validation.

This creates a potential problem if application treats token value as an identifier.

For example ES256 signature has length of 64 bytes and two padding symbols (stripped by default).
Therefore its base64-encoded value can only end with A, Q, g and w.
In non-strict mode last symbol could be tweaked resulting in 16 distinct
token values having the same signature and passing validation.

This change adds backward-compatible global config variable DecodeStrict
(similar to existing DecodePaddingAllowed) that enables strict base64 decoder mode.

See also golang/go#15656.

Signed-off-by: Alexander Yastrebov <yastrebov.alex@gmail.com>