help: improve styling of generic help text
help: improve styling of generic help text

Format the usage examples more consistently (particularly parentheses
instead of square brackets where one of several alternatives has to be
picked). And add the now much more useful `brew help <command>`.

Closes #113.

Signed-off-by: Martin Afanasjew <54669547a225ff20cba8b75a4adca540eef25858@afanasjew.de>

MikeMcQuaid/brew,konqui/brew,staticfloat/brew,toonetown/homebrew,staticfloat/brew,gregory-nisbet/brew,sjackman/homebrew,ilovezfs/brew,mahori/brew,pseudocody/brew,claui/brew,nandub/brew,tonyg/homebrew,palxex/brew,Homebrew/brew,vitorgalvao/brew,alyssais/brew,maxim-belkin/brew,toonetown/homebrew,claui/brew,hanxue/linuxbrew,Homebrew/brew,amar-laksh/brew_sudo,gordonmcshane/homebrew,hanxue/linuxbrew,nandub/brew,reitermarkus/brew,rwhogg/brew,vitorgalvao/brew,EricFromCanada/brew,rwhogg/brew,AnastasiaSulyagina/brew,mgrimes/brew,palxex/brew,reitermarkus/brew,aw1621107/brew,JCount/brew,claui/brew,gordonmcshane/homebrew,alyssais/brew,toonetown/homebrew,sjackman/homebrew,amar-laksh/brew_sudo,zmwangx/brew,pseudocody/brew,pseudocody/brew,MikeMcQuaid/brew,EricFromCanada/brew,nandub/brew,EricFromCanada/brew,mahori/brew,JCount/brew,maxim-belkin/brew,jmsundar/brew,maxim-belkin/brew,bfontaine/brew,mahori/brew,rwhogg/brew,JCount/brew,konqui/brew,jmsundar/brew,Linuxbrew/brew,zmwangx/brew,muellermartin/dist,AnastasiaSulyagina/brew,EricFromCanada/brew,nandub/brew,Homebrew/brew,Linuxbrew/brew,muellermartin/dist,AnastasiaSulyagina/brew,gregory-nisbet/brew,reelsense/brew,staticfloat/brew,muellermartin/dist,Linuxbrew/brew,sjackman/homebrew,vitorgalvao/brew,MikeMcQuaid/brew,tonyg/homebrew,MikeMcQuaid/brew,sjackman/homebrew,JCount/brew,zmwangx/brew,jmsundar/brew,reelsense/brew,aw1621107/brew,palxex/brew,bfontaine/brew,Linuxbrew/brew,mahori/brew,reitermarkus/brew,mgrimes/brew,DomT4/brew,bfontaine/brew,gordonmcshane/homebrew,Homebrew/brew,reelsense/brew,aw1621107/brew,DomT4/brew,vitorgalvao/brew,DomT4/brew,amar-laksh/brew_sudo,reitermarkus/brew,ilovezfs/brew,DomT4/brew,alyssais/brew,gregory-nisbet/brew,claui/brew,hanxue/linuxbrew,konqui/brew,ilovezfs/brew,mgrimes/brew,konqui/brew,tonyg/homebrew