Add \social options: arXiv, INSPIRE (#185)

* Add \social options: arXiv, INSPIRE

https://arxiv.org/ and https://inspirehep.net/ are websites on which
academics often have profiles that would be linked in a CV (particularly
physics and related fields).

Add social options through the academicons font.

* fixup arxiv and inspirehep socials

* fixup arxiv and inspire-hep socials

moderncviconsletters.sty

@@ -56,6 +56,8 @@
 \renewcommand*{\xboxsocialsymbol}         {\textbf{xb}~}
 \renewcommand*{\playstationsocialsymbol}  {\textbf{psn}~}
 \renewcommand*{\battlenetsocialsymbol}    {\textbf{bn}~}
+\renewcommand*{\arxivsocialsymbol}        {\textbf{arx}~}
+\renewcommand*{\inspiresocialsymbol}      {\textbf{ins}~}
 
 \renewcommand*{\listitemsymbol}      {\labelitemi~}