Display hex logo at 120px width

[peterdesmet]

Roxygen currently checks if there is a (hex) logo available and adds this to the name-package.Rd file as:

\if{html}{\figure{logo.png}{options: align='right'}}

Positioning the logo on the right is also how pkgdown instructs to add the logo to the README (and thus pkgdown homepage), but there the logo also gets a width of 120px, so bigger logos still show nicely:

---
output: github_document
---

# pkgdown <img src="man/figures/logo.png" align="right" alt="" width="120" />

This PR:

• Also sets the width at 120px, so logos display nicely independent of their size when doing ?package
• Sets an alt='logo'. This is in contrast with pkgdown where alt is empty (which I consider an oversight)

Note: I wasn't sure if the syntax for options was space or comma separated. I used space.

[hadley]

Looks good! Can you please add a bullet to NEWS? It should briefly describe the change and end with (@yourname, #issuenumber).

[peterdesmet]

@hadley thanks! Change is now described in NEWS.md: 7b2dd40

[hadley]

Thanks!