Add aplha, beta and rc bumps

[mckomo]

Changes:

• Add alpha, beta, rc bumps compatible with https://semver.org
• Refactor require statemets to allow test Rake tasks locally

Usage:

• bump alpha produces following versions 1.2.3 => 1.2.3-alpha, 1.2.3-alpha => 1.2.3-alpha.1, 1.2.3-alpha.10 => 1.2.3-alpha.11
• bump beta produces following versions 1.2.3 => 1.2.3-beta, 1.2.3-alpha => 1.2.3-beta, 1.2.3-beta => 1.2.3-beta.1, 1.2.3-beta.10 => 1.2.3-beta.11
• bump rc produces following versions 1.2.3 => 1.2.3-rc, 1.2.3-beta => 1.2.3-rc, 1.2.3-rc => 1.2.3-rc.1, 1.2.3-rc.10 => 1.2.3-rc.11
• bump pre produces following versions 1.2.3.alpha.21 => 1.2.3-beta, 1.2.3.beta.1 => 1.2.3-rc, 1.2.3.rc => 1.2.3-rc

Note: You cannot downgrade prerelease versions (e.g. from beta to alpha).

➜  bin/bump beta
Bump version 0.7.0 to 0.7.0-beta
➜  bin/bump alpha
../bump/lib/bump.rb:226:in `next_prerelease': Cannot bump prerelease version from beta to alpha (RuntimeError)

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This PR is addressing #78

[grosser]

I'm more on the 👎 side of things since:

• the only alpha bump I'd do is 1.2.3 -> 1.3.0.alpha / 1.2.3 -> 2.0.0.alpha
• it's not that hard to do them manually
• very few projects need alpha/beta/rc bumps

[mckomo]

I came to the conclusion that you are totally right with 1 point.